diff --git a/LehrerApp.Core/Services/AppLogger.cs b/LehrerApp.Core/Services/AppLogger.cs
new file mode 100644
index 0000000..8eb4aaf
--- /dev/null
+++ b/LehrerApp.Core/Services/AppLogger.cs
@@ -0,0 +1,77 @@
+namespace LehrerApp.Core.Services;
+
+public enum LogLevel { Info, Warning, Error }
+
+/// <summary>
+/// Schreibt Log-Zeilen in eine tägliche Datei im App-Datenverzeichnis
+/// (<c>&lt;AppData&gt;/LehrerApp/logs/lehrerapp-JJJJ-MM-TT.log</c>).
+/// Beim Start werden Dateien entfernt, die älter als <see cref="RetentionDays"/> sind.
+/// </summary>
+public class AppLogger
+{
+    private const int RetentionDays = 14;
+    private readonly object _lock = new();
+    private readonly string _logDirectory;
+
+    public string LogDirectory => _logDirectory;
+
+    public AppLogger(string appDataPath)
+    {
+        _logDirectory = Path.Combine(appDataPath, "logs");
+        Directory.CreateDirectory(_logDirectory);
+        RotateOldFiles();
+    }
+
+    public void Info(string message) => Write(LogLevel.Info, message, null);
+    public void Warn(string message) => Write(LogLevel.Warning, message, null);
+    public void Error(string message, Exception? exception = null) => Write(LogLevel.Error, message, exception);
+
+    private void Write(LogLevel level, string message, Exception? exception)
+    {
+        var levelLabel = level switch
+        {
+            LogLevel.Info    => "INFO ",
+            LogLevel.Warning => "WARN ",
+            LogLevel.Error   => "ERROR",
+            _                => "?    ",
+        };
+        var line = $"{DateTime.Now:yyyy-MM-dd HH:mm:ss.fff} [{levelLabel}] {message}";
+        if (exception is not null)
+            line += Environment.NewLine + exception;
+
+        lock (_lock)
+        {
+            try
+            {
+                File.AppendAllText(CurrentLogFile(), line + Environment.NewLine);
+            }
+            catch
+            {
+                // Logging darf die App nie zum Absturz bringen (z.B. Datenträger voll/gesperrt).
+            }
+        }
+    }
+
+    private string CurrentLogFile() =>
+        Path.Combine(_logDirectory, $"lehrerapp-{DateTime.Now:yyyy-MM-dd}.log");
+
+    private void RotateOldFiles()
+    {
+        var cutoff = DateTime.Now.AddDays(-RetentionDays);
+        try
+        {
+            foreach (var file in Directory.GetFiles(_logDirectory, "lehrerapp-*.log"))
+            {
+                if (File.GetLastWriteTime(file) < cutoff)
+                {
+                    try { File.Delete(file); }
+                    catch { /* nächster Start versucht es erneut */ }
+                }
+            }
+        }
+        catch
+        {
+            // Verzeichnis nicht lesbar o.ä. — Logging soll trotzdem weiterlaufen können.
+        }
+    }
+}
diff --git a/LehrerApp.Desktop/App.axaml.cs b/LehrerApp.Desktop/App.axaml.cs
index 74a5975..7bf84b7 100644
--- a/LehrerApp.Desktop/App.axaml.cs
+++ b/LehrerApp.Desktop/App.axaml.cs
@@ -1,6 +1,8 @@
 using Avalonia;
 using Avalonia.Controls.ApplicationLifetimes;
 using Avalonia.Markup.Xaml;
+using LehrerApp.Core.Services;
+using LehrerApp.Desktop.Services;
 using LehrerApp.Desktop.ViewModels;
 using LehrerApp.Desktop.ViewModels.Groups;
 using LehrerApp.Desktop.ViewModels.Students;
@@ -18,6 +20,8 @@ public class App : Application
     public override void OnFrameworkInitializationCompleted()
     {
         Services = AppBootstrapper.BuildServices();
+        Services.GetRequiredService<AppLogger>().Info("Anwendung gestartet.");
+        GlobalExceptionHandler.AttachNotifications(Services.GetRequiredService<NotificationService>());
 
         if (ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktop)
         {
diff --git a/LehrerApp.Desktop/AppBootstrapper.cs b/LehrerApp.Desktop/AppBootstrapper.cs
index d26c674..31b58c1 100644
--- a/LehrerApp.Desktop/AppBootstrapper.cs
+++ b/LehrerApp.Desktop/AppBootstrapper.cs
@@ -2,6 +2,7 @@ using LehrerApp.Core.Interfaces;
 using LehrerApp.Core.Services;
 using LehrerApp.Data;
 using LehrerApp.Data.Repositories;
+using LehrerApp.Desktop.Services;
 using LehrerApp.Desktop.ViewModels;
 using LehrerApp.Desktop.ViewModels.Groups;
 using LehrerApp.Desktop.ViewModels.Settings;
@@ -24,20 +25,44 @@ public static class AppBootstrapper
     public static string DbPath      { get; private set; } = "";
     public static string AppDataPath { get; private set; } = "";
 
+    /// <summary>
+    /// Vor der eigentlichen DI-Konfiguration verfügbar (z.B. für den globalen
+    /// Exception-Handler in Program.cs, der schon vor <see cref="BuildServices"/> greifen muss).
+    /// </summary>
+    public static AppLogger Logger { get; private set; } = null!;
+
+    public static string ResolveAppDataPath()
+    {
+        if (!string.IsNullOrEmpty(AppDataPath)) return AppDataPath;
+        var appData = Path.Combine(
+            Environment.GetFolderPath(Environment.SpecialFolder.ApplicationData),
+            "LehrerApp");
+        Directory.CreateDirectory(appData);
+        AppDataPath = appData;
+        return appData;
+    }
+
+    public static AppLogger EnsureLogger()
+    {
+        Logger ??= new AppLogger(ResolveAppDataPath());
+        return Logger;
+    }
+
     public static IServiceProvider BuildServices()
     {
         var services = new ServiceCollection();
 
         // ── Pfade ─────────────────────────────────────────────────────────────
-        var appData = Path.Combine(
-            Environment.GetFolderPath(Environment.SpecialFolder.ApplicationData),
-            "LehrerApp");
-        Directory.CreateDirectory(appData);
-        AppDataPath   = appData;
+        var appData = ResolveAppDataPath();
         DbPath        = Path.Combine(appData, "lehrerapp.db");
         var queuePath = Path.Combine(appData, "syncqueue.db");
         var keyPath   = Path.Combine(appData, "sync.key");
 
+        // ── Logging & Benachrichtigungen ─────────────────────────────────────────
+        EnsureLogger();
+        services.AddSingleton(Logger);
+        services.AddSingleton<NotificationService>();
+
         // ── Datenbank ─────────────────────────────────────────────────────────
         services.AddSingleton(_ => new LiteDbContext(DbPath));
 
diff --git a/LehrerApp.Desktop/Program.cs b/LehrerApp.Desktop/Program.cs
index acd39a9..e7a1814 100644
--- a/LehrerApp.Desktop/Program.cs
+++ b/LehrerApp.Desktop/Program.cs
@@ -1,12 +1,28 @@
 using Avalonia;
+using LehrerApp.Desktop.Services;
 
 namespace LehrerApp.Desktop;
 
 class Program
 {
     [STAThread]
-    public static void Main(string[] args) =>
-        BuildAvaloniaApp().StartWithClassicDesktopLifetime(args);
+    public static void Main(string[] args)
+    {
+        var logger = AppBootstrapper.EnsureLogger();
+        GlobalExceptionHandler.Install(logger);
+
+        try
+        {
+            BuildAvaloniaApp().StartWithClassicDesktopLifetime(args);
+        }
+        catch (Exception ex)
+        {
+            // Fehler, die den Avalonia-Nachrichtenloop selbst verlassen (z.B. beim Start),
+            // laufen hier zusammen — protokollieren, bevor der Prozess unvermeidlich endet.
+            logger.Error("Unbehandelter Fehler hat die Anwendung beendet.", ex);
+            throw;
+        }
+    }
 
     public static AppBuilder BuildAvaloniaApp() =>
         AppBuilder.Configure<App>()
diff --git a/LehrerApp.Desktop/Services/GlobalExceptionHandler.cs b/LehrerApp.Desktop/Services/GlobalExceptionHandler.cs
new file mode 100644
index 0000000..02a788d
--- /dev/null
+++ b/LehrerApp.Desktop/Services/GlobalExceptionHandler.cs
@@ -0,0 +1,55 @@
+using Avalonia.Threading;
+using LehrerApp.Core.Services;
+
+namespace LehrerApp.Desktop.Services;
+
+/// <summary>
+/// Zentrale Exception-Behandlung (13.2.1): protokolliert jeden unerwarteten Fehler und zeigt
+/// eine verständliche Meldung statt eines rohen Absturzes.
+///
+/// Reichweite ehrlich betrachtet: <see cref="Dispatcher"/>.UIThread.UnhandledException kann
+/// Ausnahmen aus Befehlen/Ereignis-Handlern auf dem UI-Thread abfangen und die App am Leben
+/// erhalten (Handled = true) — das deckt den weit überwiegenden Teil realer Abstürze in einer
+/// Desktop-App ab. AppDomain.UnhandledException und TaskScheduler.UnobservedTaskException sind
+/// Sicherheitsnetze für Fehler außerhalb des UI-Threads; die App kann eine "IsTerminating"-
+/// Ausnahme dort nicht mehr verhindern, aber wenigstens vollständig protokollieren, bevor sie endet.
+/// </summary>
+public static class GlobalExceptionHandler
+{
+    private static AppLogger? _logger;
+    private static NotificationService? _notifications;
+
+    public static void Install(AppLogger logger, NotificationService? notifications = null)
+    {
+        _logger = logger;
+        _notifications = notifications;
+
+        Dispatcher.UIThread.UnhandledException += OnDispatcherUnhandledException;
+        AppDomain.CurrentDomain.UnhandledException += OnAppDomainUnhandledException;
+        TaskScheduler.UnobservedTaskException += OnUnobservedTaskException;
+    }
+
+    /// Muss aufgerufen werden, sobald der DI-Container steht, damit spätere Fehler
+    /// (die vor dem Container auftreten könnten) noch als Toast sichtbar werden.
+    public static void AttachNotifications(NotificationService notifications) => _notifications = notifications;
+
+    private static void OnDispatcherUnhandledException(object? sender, DispatcherUnhandledExceptionEventArgs e)
+    {
+        _logger?.Error("Unbehandelter Fehler auf dem UI-Thread.", e.Exception);
+        _notifications?.ShowError("Es ist ein unerwarteter Fehler aufgetreten. Details wurden protokolliert.");
+        e.Handled = true; // App am Leben halten statt abzustürzen.
+    }
+
+    private static void OnAppDomainUnhandledException(object sender, UnhandledExceptionEventArgs e)
+    {
+        _logger?.Error(
+            $"Unbehandelter Fehler außerhalb des UI-Threads (IsTerminating={e.IsTerminating}).",
+            e.ExceptionObject as Exception);
+    }
+
+    private static void OnUnobservedTaskException(object? sender, UnobservedTaskExceptionEventArgs e)
+    {
+        _logger?.Error("Unbeobachtete Ausnahme in einem Hintergrund-Task.", e.Exception);
+        e.SetObserved(); // verhindert, dass die Ausnahme unbemerkt verschwindet.
+    }
+}
diff --git a/LehrerApp.Desktop/Services/NotificationService.cs b/LehrerApp.Desktop/Services/NotificationService.cs
new file mode 100644
index 0000000..041dca7
--- /dev/null
+++ b/LehrerApp.Desktop/Services/NotificationService.cs
@@ -0,0 +1,51 @@
+using Avalonia.Threading;
+using CommunityToolkit.Mvvm.ComponentModel;
+using System.Collections.ObjectModel;
+
+namespace LehrerApp.Desktop.Services;
+
+public enum NotificationKind { Success, Error }
+
+/// <summary>
+/// Einheitliche Erfolgs-/Fehlermeldungen als Toast (13.2.3). Wird im MainWindow als
+/// Overlay über der aktuellen Seite angezeigt und blendet sich nach kurzer Zeit selbst aus.
+/// </summary>
+public class NotificationService
+{
+    public ObservableCollection<ToastItem> Toasts { get; } = [];
+
+    public void ShowSuccess(string message) => Show(message, NotificationKind.Success, TimeSpan.FromSeconds(3));
+    public void ShowError(string message)   => Show(message, NotificationKind.Error,   TimeSpan.FromSeconds(6));
+
+    private void Show(string message, NotificationKind kind, TimeSpan duration)
+    {
+        void AddAndScheduleRemoval()
+        {
+            var toast = new ToastItem(message, kind);
+            Toasts.Add(toast);
+            _ = RemoveAfter(toast, duration);
+        }
+
+        if (Dispatcher.UIThread.CheckAccess()) AddAndScheduleRemoval();
+        else Dispatcher.UIThread.Post(AddAndScheduleRemoval);
+    }
+
+    private async Task RemoveAfter(ToastItem toast, TimeSpan duration)
+    {
+        await Task.Delay(duration);
+        Toasts.Remove(toast);
+    }
+}
+
+public partial class ToastItem : ObservableObject
+{
+    public string Message { get; }
+    public NotificationKind Kind { get; }
+    public bool IsError => Kind == NotificationKind.Error;
+
+    public ToastItem(string message, NotificationKind kind)
+    {
+        Message = message;
+        Kind = kind;
+    }
+}
diff --git a/LehrerApp.Desktop/ViewModels/Groups/ExamViewModels.cs b/LehrerApp.Desktop/ViewModels/Groups/ExamViewModels.cs
index 44daa37..0c83b17 100644
--- a/LehrerApp.Desktop/ViewModels/Groups/ExamViewModels.cs
+++ b/LehrerApp.Desktop/ViewModels/Groups/ExamViewModels.cs
@@ -28,7 +28,9 @@ public partial class ExamDialogViewModel : ObservableObject
     [ObservableProperty] private int? _examNumber;
     [ObservableProperty] private string _notes = "";
     [ObservableProperty] private string _returnedAtText = "";
-    [ObservableProperty] private string _validationMessage = "";
+    [ObservableProperty] private string _titleError = "";
+    [ObservableProperty] private string _dateTextError = "";
+    [ObservableProperty] private string _returnedAtTextError = "";
     [ObservableProperty] private double _totalPoints;
     [ObservableProperty] private bool _hasCompetencyCatalog;
     [ObservableProperty] private bool _useWeighting;
@@ -264,27 +266,34 @@ public partial class ExamDialogViewModel : ObservableObject
     [RelayCommand]
     private void Save()
     {
-        if (string.IsNullOrWhiteSpace(Title)) { ValidationMessage = "Titel erforderlich."; return; }
-        if (!DateOnly.TryParseExact(DateText, "dd.MM.yyyy", null, DateTimeStyles.None, out var date))
+        TitleError = ""; DateTextError = ""; ReturnedAtTextError = ""; GradingKeyValidation = "";
+        var valid = true;
+
+        if (string.IsNullOrWhiteSpace(Title)) { TitleError = "Titel erforderlich."; valid = false; }
+
+        DateOnly date = default;
+        if (!DateOnly.TryParseExact(DateText, "dd.MM.yyyy", null, DateTimeStyles.None, out date))
         {
-            ValidationMessage = "Datum im Format TT.MM.JJJJ eingeben.";
-            return;
+            DateTextError = "Format TT.MM.JJJJ.";
+            valid = false;
         }
+
         DateOnly? returnedAt = null;
         if (!string.IsNullOrWhiteSpace(ReturnedAtText))
         {
             if (!DateOnly.TryParseExact(ReturnedAtText, "dd.MM.yyyy", null, DateTimeStyles.None, out var r))
             {
-                ValidationMessage = "Rückgabedatum im Format TT.MM.JJJJ eingeben.";
-                return;
+                ReturnedAtTextError = "Format TT.MM.JJJJ.";
+                valid = false;
             }
-            returnedAt = r;
+            else returnedAt = r;
         }
 
         var gradingKey = BuildGradingKeyEntries();
         var gradingKeyError = _grading.ValidateGradingKey(gradingKey);
-        if (gradingKeyError is not null) { GradingKeyValidation = gradingKeyError; return; }
-        GradingKeyValidation = "";
+        if (gradingKeyError is not null) { GradingKeyValidation = gradingKeyError; valid = false; }
+
+        if (!valid) return;
 
         Result = _editingExam ?? new Exam { GroupId = _groupId };
         Result.Title      = Title.Trim();
diff --git a/LehrerApp.Desktop/ViewModels/Groups/GradeOverviewViewModels.cs b/LehrerApp.Desktop/ViewModels/Groups/GradeOverviewViewModels.cs
index 393a1ec..2a1f725 100644
--- a/LehrerApp.Desktop/ViewModels/Groups/GradeOverviewViewModels.cs
+++ b/LehrerApp.Desktop/ViewModels/Groups/GradeOverviewViewModels.cs
@@ -315,11 +315,16 @@ public partial class StudentGradesDialogViewModel : ObservableObject
 
     private void Save(GradeEditItem item)
     {
-        if (string.IsNullOrWhiteSpace(item.Value)) { item.ValidationMessage = "Wert darf nicht leer sein."; return; }
+        item.ValueError = ""; item.DateTextError = "";
+        var valid = true;
+
+        if (string.IsNullOrWhiteSpace(item.Value)) { item.ValueError = "Wert darf nicht leer sein."; valid = false; }
         if (!DateOnly.TryParseExact(item.DateText, "dd.MM.yyyy", null,
                 System.Globalization.DateTimeStyles.None, out _))
-        { item.ValidationMessage = "Datum im Format TT.MM.JJJJ eingeben."; return; }
-        item.ValidationMessage = "";
+        { item.DateTextError = "Format TT.MM.JJJJ."; valid = false; }
+
+        if (!valid) return;
+
         var grade = item.ToModel();
         _grades.Save(grade);
         item.IsNew = false;
@@ -345,7 +350,8 @@ public partial class GradeEditItem : ObservableObject
     [ObservableProperty] private string _dateText;
     [ObservableProperty] private double _weight;
     [ObservableProperty] private string? _note;
-    [ObservableProperty] private string _validationMessage = "";
+    [ObservableProperty] private string _valueError = "";
+    [ObservableProperty] private string _dateTextError = "";
 
     // Für die ComboBox: deutsche Anzeige statt des rohen Enum-Namens.
     public string CategoryName
@@ -405,7 +411,7 @@ public partial class CollectiveGradeDialogViewModel : ObservableObject
     [ObservableProperty] private double _weight = 1.0;
     [ObservableProperty] private string? _note;
     [ObservableProperty] private string _statusMessage = "";
-    [ObservableProperty] private string _validationMessage = "";
+    [ObservableProperty] private string _dateTextError = "";
 
     // Für die ComboBox: deutsche Anzeige statt des rohen Enum-Namens.
     public string CategoryName
@@ -428,8 +434,8 @@ public partial class CollectiveGradeDialogViewModel : ObservableObject
     {
         if (!DateOnly.TryParseExact(DateText, "dd.MM.yyyy", null,
                 System.Globalization.DateTimeStyles.None, out var date))
-        { ValidationMessage = "Datum im Format TT.MM.JJJJ eingeben."; return; }
-        ValidationMessage = "";
+        { DateTextError = "Format TT.MM.JJJJ."; return; }
+        DateTextError = "";
 
         var count = 0;
         foreach (var row in Rows)
diff --git a/LehrerApp.Desktop/ViewModels/Groups/GroupViewModels.cs b/LehrerApp.Desktop/ViewModels/Groups/GroupViewModels.cs
index 337ba7c..017ab39 100644
--- a/LehrerApp.Desktop/ViewModels/Groups/GroupViewModels.cs
+++ b/LehrerApp.Desktop/ViewModels/Groups/GroupViewModels.cs
@@ -618,7 +618,8 @@ public partial class AddGroupDialogViewModel : ObservableObject
     [ObservableProperty] private int? _hoursPerWeek;
     [ObservableProperty] private bool _isOwnClass;
     [ObservableProperty] private bool _isDifferentiated;
-    [ObservableProperty] private string _validationMessage = "";
+    [ObservableProperty] private string _nameError = "";
+    [ObservableProperty] private string _gradeLevelError = "";
 
     public List<string> SchoolYears { get; }
     public List<string> GradingOptions { get; } = ["Noten 1–6", "Punkte 0–15"];
@@ -662,8 +663,13 @@ public partial class AddGroupDialogViewModel : ObservableObject
     [RelayCommand]
     private void Save()
     {
-        if (string.IsNullOrWhiteSpace(Name)) { ValidationMessage = "Bezeichnung erforderlich."; return; }
-        if (GradeLevel is < 1 or > 13)       { ValidationMessage = "Klassenstufe 1–13."; return; }
+        NameError = ""; GradeLevelError = "";
+        var valid = true;
+
+        if (string.IsNullOrWhiteSpace(Name)) { NameError = "Bezeichnung erforderlich."; valid = false; }
+        if (GradeLevel is < 1 or > 13)       { GradeLevelError = "Klassenstufe 1–13."; valid = false; }
+
+        if (!valid) return;
 
         string? subjectName = !string.IsNullOrWhiteSpace(Subject) ? Subject.Trim() : null;
         Guid?   subjectId   = null;
diff --git a/LehrerApp.Desktop/ViewModels/Groups/ParticipationViewModels.cs b/LehrerApp.Desktop/ViewModels/Groups/ParticipationViewModels.cs
index e349b5b..8244ca7 100644
--- a/LehrerApp.Desktop/ViewModels/Groups/ParticipationViewModels.cs
+++ b/LehrerApp.Desktop/ViewModels/Groups/ParticipationViewModels.cs
@@ -671,7 +671,7 @@ public partial class AddSessionDialogViewModel : ObservableObject
     [ObservableProperty] private DateOnly _date = DateOnly.FromDateTime(DateTime.Today);
     [ObservableProperty] private string   _comment = "";
     [ObservableProperty] private string   _dateText = DateOnly.FromDateTime(DateTime.Today).ToString("dd.MM.yyyy");
-    [ObservableProperty] private string   _validationMessage = "";
+    [ObservableProperty] private string   _dateTextError = "";
 
     public ParticipationSession? Result { get; private set; }
 
@@ -681,9 +681,10 @@ public partial class AddSessionDialogViewModel : ObservableObject
         if (!DateOnly.TryParseExact(DateText, "dd.MM.yyyy", null,
                 System.Globalization.DateTimeStyles.None, out var date))
         {
-            ValidationMessage = "Datum im Format TT.MM.JJJJ eingeben.";
+            DateTextError = "Format TT.MM.JJJJ.";
             return;
         }
+        DateTextError = "";
         Result = new ParticipationSession { Date = date, Comment = Comment.Trim() };
     }
 }
diff --git a/LehrerApp.Desktop/ViewModels/Groups/ParticipationWizardViewModels.cs b/LehrerApp.Desktop/ViewModels/Groups/ParticipationWizardViewModels.cs
index e439c99..0720e27 100644
--- a/LehrerApp.Desktop/ViewModels/Groups/ParticipationWizardViewModels.cs
+++ b/LehrerApp.Desktop/ViewModels/Groups/ParticipationWizardViewModels.cs
@@ -37,7 +37,8 @@ public partial class ParticipationWizardDialogViewModel : ObservableObject
     [ObservableProperty] private string _progressText = "";
     [ObservableProperty] private string _newSectionLabel = "";
     [ObservableProperty] private string _newSectionEndDateText = DateOnly.FromDateTime(DateTime.Today).ToString("dd.MM.yyyy");
-    [ObservableProperty] private string _sectionValidationMessage = "";
+    [ObservableProperty] private string _newSectionLabelError = "";
+    [ObservableProperty] private string _newSectionEndDateTextError = "";
     [ObservableProperty] private ParticipationPeriodOption _rollupPeriod;
     [ObservableProperty] private string _rollupStatusMessage = "";
     [ObservableProperty] private double _timelineZoom = 1.0;
@@ -322,13 +323,20 @@ public partial class ParticipationWizardDialogViewModel : ObservableObject
     [RelayCommand]
     private void CloseSection()
     {
-        if (string.IsNullOrWhiteSpace(NewSectionLabel)) { SectionValidationMessage = "Bezeichnung erforderlich."; return; }
+        NewSectionLabelError = ""; NewSectionEndDateTextError = "";
+        var valid = true;
+
+        if (string.IsNullOrWhiteSpace(NewSectionLabel)) { NewSectionLabelError = "Bezeichnung erforderlich."; valid = false; }
+
+        DateOnly end = default;
         if (!DateOnly.TryParseExact(NewSectionEndDateText, "dd.MM.yyyy", null,
-                DateTimeStyles.None, out var end))
-        { SectionValidationMessage = "Datum im Format TT.MM.JJJJ eingeben."; return; }
+                DateTimeStyles.None, out end))
+        { NewSectionEndDateTextError = "Format TT.MM.JJJJ."; valid = false; }
+
+        if (!valid) return;
 
         var start = ComputeOpenStart();
-        if (end < start) { SectionValidationMessage = "Enddatum liegt vor Abschnittsbeginn."; return; }
+        if (end < start) { NewSectionEndDateTextError = "Enddatum liegt vor Abschnittsbeginn."; return; }
 
         var section = new ParticipationSection { GroupId = _groupId, Label = NewSectionLabel.Trim(), StartDate = start, EndDate = end };
         _sectionRepo.Save(section);
@@ -350,7 +358,6 @@ public partial class ParticipationWizardDialogViewModel : ObservableObject
             });
         }
 
-        SectionValidationMessage = "";
         NewSectionLabel = $"Abschnitt {_sectionList.Count + 1}";
         BuildTimeline(CurrentStudentId);
         BuildSections(CurrentStudentId);
diff --git a/LehrerApp.Desktop/ViewModels/MainWindowViewModel.cs b/LehrerApp.Desktop/ViewModels/MainWindowViewModel.cs
index 00cf914..e33ceba 100644
--- a/LehrerApp.Desktop/ViewModels/MainWindowViewModel.cs
+++ b/LehrerApp.Desktop/ViewModels/MainWindowViewModel.cs
@@ -1,10 +1,12 @@
 using CommunityToolkit.Mvvm.ComponentModel;
 using CommunityToolkit.Mvvm.Input;
 using LehrerApp.Core.Services;
+using LehrerApp.Desktop.Services;
 using LehrerApp.Desktop.ViewModels.Groups;
 using LehrerApp.Desktop.ViewModels.Settings;
 using LehrerApp.Desktop.ViewModels.Students;
 using Microsoft.Extensions.DependencyInjection;
+using System.Collections.ObjectModel;
 
 namespace LehrerApp.Desktop.ViewModels;
 
@@ -17,13 +19,15 @@ public partial class MainWindowViewModel : ObservableObject
     [ObservableProperty] private string _currentSchoolYear = "";
 
     public SyncStatusViewModel SyncStatus { get; }
+    public ObservableCollection<ToastItem> Toasts { get; }
 
     public MainWindowViewModel(IServiceProvider services,
         DashboardViewModel dashboard, SchoolYearService sy,
-        SyncStatusViewModel syncStatus)
+        SyncStatusViewModel syncStatus, NotificationService notifications)
     {
         _services         = services;
         SyncStatus        = syncStatus;
+        Toasts            = notifications.Toasts;
         CurrentSchoolYear = sy.CurrentSchoolYear();
         CurrentPage       = dashboard;
     }
diff --git a/LehrerApp.Desktop/ViewModels/Settings/SettingsViewModel.cs b/LehrerApp.Desktop/ViewModels/Settings/SettingsViewModel.cs
index 2dea95a..601d8c7 100644
--- a/LehrerApp.Desktop/ViewModels/Settings/SettingsViewModel.cs
+++ b/LehrerApp.Desktop/ViewModels/Settings/SettingsViewModel.cs
@@ -23,7 +23,7 @@ public partial class SettingsViewModel : ObservableObject
 
     [ObservableProperty] private string _newName          = "";
     [ObservableProperty] private string _newShort         = "";
-    [ObservableProperty] private string _validationMessage = "";
+    [ObservableProperty] private string _newNameError     = "";
 
     public ObservableCollection<SubjectListItem> Subjects { get; } = [];
 
@@ -41,7 +41,7 @@ public partial class SettingsViewModel : ObservableObject
 
     [ObservableProperty] private string _newTemplateName = "";
     [ObservableProperty] private string _newTemplateGradingSystemName = "Noten 1–6";
-    [ObservableProperty] private string _templateValidationMessage = "";
+    [ObservableProperty] private string _newTemplateNameError = "";
 
     public List<string> GradingSystemOptions { get; } = ["Noten 1–6", "Punkte 0–15"];
     public ObservableCollection<GradingKeyTemplateEditItem> GradingKeyTemplateList { get; } = [];
@@ -93,7 +93,7 @@ public partial class SettingsViewModel : ObservableObject
     [RelayCommand]
     private void AddGradingKeyTemplate()
     {
-        if (string.IsNullOrWhiteSpace(NewTemplateName)) { TemplateValidationMessage = "Vorlagenname erforderlich."; return; }
+        if (string.IsNullOrWhiteSpace(NewTemplateName)) { NewTemplateNameError = "Vorlagenname erforderlich."; return; }
 
         var system = NewTemplateGradingSystemName == "Punkte 0–15"
             ? GradingSystem.Points0To15 : GradingSystem.Grades1To6;
@@ -109,7 +109,7 @@ public partial class SettingsViewModel : ObservableObject
         _gradingKeyTemplates.Save(template);
         GradingKeyTemplateList.Add(new GradingKeyTemplateEditItem(template, _gradingKeyTemplates, _grading));
         NewTemplateName = "";
-        TemplateValidationMessage = "";
+        NewTemplateNameError = "";
     }
 
     [RelayCommand]
@@ -132,17 +132,17 @@ public partial class SettingsViewModel : ObservableObject
     [RelayCommand]
     private void AddSubject()
     {
-        if (string.IsNullOrWhiteSpace(NewName)) { ValidationMessage = "Name erforderlich."; return; }
+        if (string.IsNullOrWhiteSpace(NewName)) { NewNameError = "Name erforderlich."; return; }
         try
         {
             _subjects.Save(new Subject { Name = NewName.Trim(), ShortName = NewShort.Trim() });
         }
         catch (InvalidOperationException ex)
         {
-            ValidationMessage = ex.Message;
+            NewNameError = ex.Message;
             return;
         }
-        NewName = ""; NewShort = ""; ValidationMessage = "";
+        NewName = ""; NewShort = ""; NewNameError = "";
         LoadSubjects();
     }
 
@@ -156,10 +156,10 @@ public partial class SettingsViewModel : ObservableObject
         }
         catch (InvalidOperationException ex)
         {
-            ValidationMessage = ex.Message;
+            NewNameError = ex.Message;
             return;
         }
-        ValidationMessage = "";
+        NewNameError = "";
         if (CatalogSubject?.Id == item.Id) CatalogSubject = null;
         LoadSubjects();
     }
@@ -362,7 +362,8 @@ public partial class GradingKeyTemplateEditItem : ObservableObject
 
     [ObservableProperty] private string _newGrade = "";
     [ObservableProperty] private double _newMinPercent;
-    [ObservableProperty] private string _validation = "";
+    [ObservableProperty] private string _newGradeError = "";
+    [ObservableProperty] private string _newMinPercentError = "";
     [ObservableProperty] private string _completenessWarning = "";
 
     public ObservableCollection<GradingKeyEntryVm> Entries { get; } = [];
@@ -384,10 +385,15 @@ public partial class GradingKeyTemplateEditItem : ObservableObject
     [RelayCommand]
     private void AddEntry()
     {
-        if (string.IsNullOrWhiteSpace(NewGrade)) { Validation = "Bezeichnung erforderlich."; return; }
-        if (NewMinPercent is < 0 or > 100) { Validation = "Prozentgrenze muss zwischen 0 und 100 liegen."; return; }
-        if (_template.Entries.Any(e => Math.Abs(e.MinPercent - NewMinPercent) < 0.0001))
-        { Validation = "Diese Prozentgrenze existiert bereits."; return; }
+        NewGradeError = ""; NewMinPercentError = "";
+        var valid = true;
+
+        if (string.IsNullOrWhiteSpace(NewGrade)) { NewGradeError = "Bezeichnung erforderlich."; valid = false; }
+        if (NewMinPercent is < 0 or > 100) { NewMinPercentError = "Muss zwischen 0 und 100 liegen."; valid = false; }
+        else if (_template.Entries.Any(e => Math.Abs(e.MinPercent - NewMinPercent) < 0.0001))
+        { NewMinPercentError = "Diese Prozentgrenze existiert bereits."; valid = false; }
+
+        if (!valid) return;
 
         var entry = new GradingKeyEntry { Grade = NewGrade.Trim(), MinPercent = NewMinPercent };
         _template.Entries.Add(entry);
@@ -396,7 +402,7 @@ public partial class GradingKeyTemplateEditItem : ObservableObject
 
         Entries.Clear();
         foreach (var e in _template.Entries) Entries.Add(new GradingKeyEntryVm(e, DeleteEntry));
-        NewGrade = ""; NewMinPercent = 0; Validation = "";
+        NewGrade = ""; NewMinPercent = 0;
         RecomputeCompleteness();
     }
 
diff --git a/LehrerApp.Desktop/ViewModels/Students/StudentViewModels.cs b/LehrerApp.Desktop/ViewModels/Students/StudentViewModels.cs
index 0293ce5..818ff39 100644
--- a/LehrerApp.Desktop/ViewModels/Students/StudentViewModels.cs
+++ b/LehrerApp.Desktop/ViewModels/Students/StudentViewModels.cs
@@ -361,7 +361,9 @@ public partial class AddStudentDialogViewModel : ObservableObject
     [ObservableProperty] private string _dateOfBirthText = "";
     [ObservableProperty] private string _selectedGender = "";
     [ObservableProperty] private string _notes = "";
-    [ObservableProperty] private string _validationMessage = "";
+    [ObservableProperty] private string _lastNameError = "";
+    [ObservableProperty] private string _firstNameError = "";
+    [ObservableProperty] private string _dateOfBirthError = "";
 
     public List<string> GenderOptions    { get; } = ["", "M – männlich", "W – weiblich", "D – divers"];
     public List<string> RelationPresets  { get; } = ["Schüler/in", "Mutter", "Vater", "Elternteil", "Erziehungsberechtigte/r", "Sonstige"];
@@ -380,8 +382,11 @@ public partial class AddStudentDialogViewModel : ObservableObject
     [RelayCommand]
     private void Save()
     {
-        if (string.IsNullOrWhiteSpace(LastName))  { ValidationMessage = "Nachname erforderlich."; return; }
-        if (string.IsNullOrWhiteSpace(FirstName)) { ValidationMessage = "Vorname erforderlich.";  return; }
+        LastNameError = ""; FirstNameError = ""; DateOfBirthError = "";
+        var valid = true;
+
+        if (string.IsNullOrWhiteSpace(LastName))  { LastNameError = "Nachname erforderlich."; valid = false; }
+        if (string.IsNullOrWhiteSpace(FirstName)) { FirstNameError = "Vorname erforderlich."; valid = false; }
 
         DateOnly? dob = null;
         if (!string.IsNullOrWhiteSpace(DateOfBirthText))
@@ -389,11 +394,13 @@ public partial class AddStudentDialogViewModel : ObservableObject
             if (!DateOnly.TryParseExact(DateOfBirthText, "dd.MM.yyyy", null,
                     System.Globalization.DateTimeStyles.None, out var d))
             {
-                ValidationMessage = "Geburtsdatum im Format TT.MM.JJJJ."; return;
+                DateOfBirthError = "Format TT.MM.JJJJ."; valid = false;
             }
-            dob = d;
+            else dob = d;
         }
 
+        if (!valid) return;
+
         Result = new Student
         {
             FirstName   = FirstName.Trim(),
@@ -456,7 +463,8 @@ public partial class ContactEditDialogViewModel : ObservableObject
     [ObservableProperty] private string _invalidSinceText = "";
     [ObservableProperty] private string _selectedInvalidReason = "";
     [ObservableProperty] private string _invalidReasonDetails = "";
-    [ObservableProperty] private string _validationMessage = "";
+    [ObservableProperty] private string _nameError = "";
+    [ObservableProperty] private string _invalidSinceError = "";
 
     public string DialogTitle => _source is null ? "Kontakt anlegen" : "Kontakt bearbeiten";
     public List<string> RelationPresets { get; } =
@@ -494,10 +502,13 @@ public partial class ContactEditDialogViewModel : ObservableObject
     [RelayCommand]
     private void Save()
     {
+        NameError = ""; InvalidSinceError = "";
+        var valid = true;
+
         if (string.IsNullOrWhiteSpace(Name))
         {
-            ValidationMessage = "Name erforderlich.";
-            return;
+            NameError = "Name erforderlich.";
+            valid = false;
         }
 
         DateOnly? invalidSince = null;
@@ -506,12 +517,14 @@ public partial class ContactEditDialogViewModel : ObservableObject
             if (!DateOnly.TryParseExact(InvalidSinceText, "dd.MM.yyyy", null,
                     System.Globalization.DateTimeStyles.None, out var parsedDate))
             {
-                ValidationMessage = "Ungültig seit im Format TT.MM.JJJJ angeben.";
-                return;
+                InvalidSinceError = "Format TT.MM.JJJJ.";
+                valid = false;
             }
-            invalidSince = parsedDate;
+            else invalidSince = parsedDate;
         }
 
+        if (!valid) return;
+
         Result = new Contact
         {
             Id = _source?.Id ?? Guid.NewGuid(),
diff --git a/LehrerApp.Desktop/Views/Groups/AddGroupDialog.axaml b/LehrerApp.Desktop/Views/Groups/AddGroupDialog.axaml
index c64dafc..ac11dfe 100644
--- a/LehrerApp.Desktop/Views/Groups/AddGroupDialog.axaml
+++ b/LehrerApp.Desktop/Views/Groups/AddGroupDialog.axaml
@@ -23,6 +23,8 @@
       <StackPanel Spacing="4">
         <TextBlock Text="{Binding NameLabel}" FontSize="12" Opacity="0.7"/>
         <TextBox Text="{Binding Name}" PlaceholderText="{Binding NameHint}"/>
+        <TextBlock Text="{Binding NameError}" Foreground="Red" FontSize="11"
+                   IsVisible="{Binding NameError, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"/>
       </StackPanel>
 
       <!-- Fach – AutoComplete aus bekannten Fächern; auch bei Klasse, da eine
@@ -41,6 +43,8 @@
         <StackPanel Grid.Column="0" Spacing="4">
           <TextBlock Text="Klassenstufe *" FontSize="12" Opacity="0.7"/>
           <NumericUpDown Value="{Binding GradeLevel}" Minimum="1" Maximum="13" FormatString="0"/>
+          <TextBlock Text="{Binding GradeLevelError}" Foreground="Red" FontSize="11"
+                     IsVisible="{Binding GradeLevelError, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"/>
         </StackPanel>
         <StackPanel Grid.Column="2" Spacing="4">
           <TextBlock Text="Schuljahr" FontSize="12" Opacity="0.7"/>
@@ -70,8 +74,6 @@
       <CheckBox Content="Niveaudifferenzierung (E/G/Förder)" IsChecked="{Binding IsDifferentiated}"
                 ToolTip.Tip="Blendet im Schüler-Tab eine Niveau-Zuordnung je Schüler ein und erlaubt in Klausuren die Zuordnung zu einem Niveau."/>
 
-      <TextBlock Text="{Binding ValidationMessage}" Foreground="Red" FontSize="12"
-                 IsVisible="{Binding ValidationMessage, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"/>
     </StackPanel>
 
     <Grid Grid.Row="1" ColumnDefinitions="*,8,*" Margin="0,20,0,0">
diff --git a/LehrerApp.Desktop/Views/Groups/AddSessionDialog.axaml b/LehrerApp.Desktop/Views/Groups/AddSessionDialog.axaml
index 00bc3b5..b7aac11 100644
--- a/LehrerApp.Desktop/Views/Groups/AddSessionDialog.axaml
+++ b/LehrerApp.Desktop/Views/Groups/AddSessionDialog.axaml
@@ -14,15 +14,14 @@
       <StackPanel Spacing="4">
         <TextBlock Text="Datum *" FontSize="12" Opacity="0.7"/>
         <TextBox Text="{Binding DateText}" PlaceholderText="TT.MM.JJJJ" x:Name="DateBox"/>
+        <TextBlock Text="{Binding DateTextError}" Foreground="Red" FontSize="11"
+                   IsVisible="{Binding DateTextError, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"/>
       </StackPanel>
 
       <StackPanel Spacing="4">
         <TextBlock Text="Kommentar (optional)" FontSize="12" Opacity="0.7"/>
         <TextBox Text="{Binding Comment}" PlaceholderText="z. B. Stunde 12 – Säure-Base-Reaktion"/>
       </StackPanel>
-
-      <TextBlock Text="{Binding ValidationMessage}" Foreground="Red" FontSize="12"
-                 IsVisible="{Binding ValidationMessage, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"/>
     </StackPanel>
 
     <Grid Grid.Row="1" ColumnDefinitions="*,8,*" Margin="0,20,0,0">
diff --git a/LehrerApp.Desktop/Views/Groups/CollectiveGradeDialog.axaml b/LehrerApp.Desktop/Views/Groups/CollectiveGradeDialog.axaml
index 08d87dc..fd460c6 100644
--- a/LehrerApp.Desktop/Views/Groups/CollectiveGradeDialog.axaml
+++ b/LehrerApp.Desktop/Views/Groups/CollectiveGradeDialog.axaml
@@ -15,14 +15,16 @@
       <Grid ColumnDefinitions="120,*,90" ColumnSpacing="6">
         <ComboBox Grid.Column="0" ItemsSource="{x:Static vm:GradeCategoryDisplay.Options}"
                   SelectedItem="{Binding CategoryName}"/>
-        <TextBox Grid.Column="1" Text="{Binding DateText}" PlaceholderText="TT.MM.JJJJ"/>
+        <StackPanel Grid.Column="1" Spacing="2">
+          <TextBox Text="{Binding DateText}" PlaceholderText="TT.MM.JJJJ"/>
+          <TextBlock Text="{Binding DateTextError}" Foreground="Red" FontSize="11"
+                     IsVisible="{Binding DateTextError, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"/>
+        </StackPanel>
         <NumericUpDown Grid.Column="2" Value="{Binding Weight}" Minimum="0" Maximum="10"
                        Increment="0.1" FormatString="0.#" ShowButtonSpinner="False"
                        ToolTip.Tip="Gewichtung"/>
       </Grid>
       <TextBox Text="{Binding Note}" PlaceholderText="Notiz (z.B. Hausaufgabenkontrolle)"/>
-      <TextBlock Text="{Binding ValidationMessage}" Foreground="Red" FontSize="12"
-                 IsVisible="{Binding ValidationMessage, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"/>
       <TextBlock Text="Werte pro Schüler unten eintragen. Leere Felder werden nicht gespeichert."
                  FontSize="11" Opacity="0.5" TextWrapping="Wrap"/>
     </StackPanel>
diff --git a/LehrerApp.Desktop/Views/Groups/ExamDialog.axaml b/LehrerApp.Desktop/Views/Groups/ExamDialog.axaml
index 3423773..fadd580 100644
--- a/LehrerApp.Desktop/Views/Groups/ExamDialog.axaml
+++ b/LehrerApp.Desktop/Views/Groups/ExamDialog.axaml
@@ -19,6 +19,8 @@
         <StackPanel Spacing="4">
           <TextBlock Text="Titel *" FontSize="12" Opacity="0.7"/>
           <TextBox Text="{Binding Title}" PlaceholderText="z.B. 1. Klausur Kinetik"/>
+          <TextBlock Text="{Binding TitleError}" Foreground="Red" FontSize="11"
+                     IsVisible="{Binding TitleError, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"/>
         </StackPanel>
 
         <StackPanel Spacing="4" IsVisible="{Binding IsDifferentiated}">
@@ -32,6 +34,8 @@
           <StackPanel Grid.Column="0" Spacing="4">
             <TextBlock Text="Datum *" FontSize="12" Opacity="0.7"/>
             <TextBox Text="{Binding DateText}" PlaceholderText="TT.MM.JJJJ"/>
+            <TextBlock Text="{Binding DateTextError}" Foreground="Red" FontSize="11"
+                       IsVisible="{Binding DateTextError, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"/>
           </StackPanel>
           <StackPanel Grid.Column="2" Spacing="4">
             <TextBlock Text="Klausurnummer" FontSize="12" Opacity="0.7"/>
@@ -41,6 +45,8 @@
           <StackPanel Grid.Column="4" Spacing="4">
             <TextBlock Text="Rückgabedatum" FontSize="12" Opacity="0.7"/>
             <TextBox Text="{Binding ReturnedAtText}" PlaceholderText="TT.MM.JJJJ"/>
+            <TextBlock Text="{Binding ReturnedAtTextError}" Foreground="Red" FontSize="11"
+                       IsVisible="{Binding ReturnedAtTextError, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"/>
           </StackPanel>
         </Grid>
 
@@ -118,9 +124,6 @@
           </ItemsControl.ItemTemplate>
         </ItemsControl>
 
-        <TextBlock Text="{Binding ValidationMessage}" Foreground="Red" FontSize="12"
-                   IsVisible="{Binding ValidationMessage, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"/>
-
         <Separator Margin="0,4"/>
 
         <Grid ColumnDefinitions="*,Auto">
diff --git a/LehrerApp.Desktop/Views/Groups/ParticipationWizardDialog.axaml b/LehrerApp.Desktop/Views/Groups/ParticipationWizardDialog.axaml
index fe1c5bd..e0cd0fb 100644
--- a/LehrerApp.Desktop/Views/Groups/ParticipationWizardDialog.axaml
+++ b/LehrerApp.Desktop/Views/Groups/ParticipationWizardDialog.axaml
@@ -128,11 +128,17 @@
         <StackPanel Spacing="6">
           <TextBlock Text="Abschnitt abschließen" FontSize="12" FontWeight="SemiBold"/>
           <Grid ColumnDefinitions="*,100" ColumnSpacing="6">
-            <TextBox Grid.Column="0" Text="{Binding NewSectionLabel}" PlaceholderText="Bezeichnung"/>
-            <TextBox Grid.Column="1" Text="{Binding NewSectionEndDateText}" PlaceholderText="TT.MM.JJJJ"/>
+            <StackPanel Grid.Column="0" Spacing="2">
+              <TextBox Text="{Binding NewSectionLabel}" PlaceholderText="Bezeichnung"/>
+              <TextBlock Text="{Binding NewSectionLabelError}" Foreground="Red" FontSize="11"
+                         IsVisible="{Binding NewSectionLabelError, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"/>
+            </StackPanel>
+            <StackPanel Grid.Column="1" Spacing="2">
+              <TextBox Text="{Binding NewSectionEndDateText}" PlaceholderText="TT.MM.JJJJ"/>
+              <TextBlock Text="{Binding NewSectionEndDateTextError}" Foreground="Red" FontSize="11"
+                         IsVisible="{Binding NewSectionEndDateTextError, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"/>
+            </StackPanel>
           </Grid>
-          <TextBlock Text="{Binding SectionValidationMessage}" Foreground="Red" FontSize="11"
-                     IsVisible="{Binding SectionValidationMessage, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"/>
           <Button Content="Abschnitt für alle Schüler abschließen" Command="{Binding CloseSectionCommand}"
                   HorizontalAlignment="Stretch"/>
         </StackPanel>
diff --git a/LehrerApp.Desktop/Views/Groups/StudentGradesDialog.axaml b/LehrerApp.Desktop/Views/Groups/StudentGradesDialog.axaml
index bfefc8f..bbe1c29 100644
--- a/LehrerApp.Desktop/Views/Groups/StudentGradesDialog.axaml
+++ b/LehrerApp.Desktop/Views/Groups/StudentGradesDialog.axaml
@@ -24,15 +24,21 @@
                 <Grid ColumnDefinitions="120,*,90,90" ColumnSpacing="6">
                   <ComboBox Grid.Column="0" ItemsSource="{x:Static vm:GradeCategoryDisplay.Options}"
                             SelectedItem="{Binding CategoryName}"/>
-                  <TextBox Grid.Column="1" Text="{Binding Value}" PlaceholderText="Wert (z.B. 2 oder 11)"/>
-                  <TextBox Grid.Column="2" Text="{Binding DateText}" PlaceholderText="TT.MM.JJJJ"/>
+                  <StackPanel Grid.Column="1" Spacing="2">
+                    <TextBox Text="{Binding Value}" PlaceholderText="Wert (z.B. 2 oder 11)"/>
+                    <TextBlock Text="{Binding ValueError}" Foreground="Red" FontSize="11"
+                               IsVisible="{Binding ValueError, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"/>
+                  </StackPanel>
+                  <StackPanel Grid.Column="2" Spacing="2">
+                    <TextBox Text="{Binding DateText}" PlaceholderText="TT.MM.JJJJ"/>
+                    <TextBlock Text="{Binding DateTextError}" Foreground="Red" FontSize="11"
+                               IsVisible="{Binding DateTextError, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"/>
+                  </StackPanel>
                   <NumericUpDown Grid.Column="3" Value="{Binding Weight}" Minimum="0" Maximum="10"
                                  Increment="0.1" FormatString="0.#" ShowButtonSpinner="False"
                                  ToolTip.Tip="Gewichtung"/>
                 </Grid>
                 <TextBox Text="{Binding Note}" PlaceholderText="Notiz (optional)" FontSize="12"/>
-                <TextBlock Text="{Binding ValidationMessage}" Foreground="Red" FontSize="11"
-                           IsVisible="{Binding ValidationMessage, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"/>
                 <Grid ColumnDefinitions="*,Auto,Auto">
                   <TextBlock Grid.Column="0" Text="{Binding CreatedAtDisplay}" FontSize="11" Opacity="0.5"
                              VerticalAlignment="Center"/>
diff --git a/LehrerApp.Desktop/Views/MainWindow.axaml b/LehrerApp.Desktop/Views/MainWindow.axaml
index b566c00..7867b94 100644
--- a/LehrerApp.Desktop/Views/MainWindow.axaml
+++ b/LehrerApp.Desktop/Views/MainWindow.axaml
@@ -9,12 +9,14 @@
         xmlns:vs="clr-namespace:LehrerApp.Desktop.Views.Students"
         xmlns:vset="clr-namespace:LehrerApp.Desktop.Views.Settings"
         xmlns:vmset="clr-namespace:LehrerApp.Desktop.ViewModels.Settings"
+        xmlns:svc="clr-namespace:LehrerApp.Desktop.Services"
         x:Class="LehrerApp.Desktop.Views.MainWindow"
         x:DataType="vm:MainWindowViewModel"
         Title="LehrerApp"
         Width="1280" Height="800"
         MinWidth="900" MinHeight="600">
 
+  <Panel>
   <!--
     Avalonia 12: DrawerPage ersetzt die selbstgebaute Sidebar.
     DrawerBreakpointWidth="900" → Sidebar ab 900px dauerhaft sichtbar,
@@ -189,4 +191,32 @@
     </DrawerPage.Drawer>
 
   </DrawerPage>
+
+  <!-- Toast-Benachrichtigungen (13.2.3): schweben über der aktuellen Seite, unten rechts. -->
+  <ItemsControl ItemsSource="{Binding Toasts}"
+                HorizontalAlignment="Right" VerticalAlignment="Bottom"
+                Margin="0,0,20,20" IsHitTestVisible="False">
+    <ItemsControl.ItemsPanel>
+      <ItemsPanelTemplate>
+        <StackPanel Spacing="8"/>
+      </ItemsPanelTemplate>
+    </ItemsControl.ItemsPanel>
+    <ItemsControl.Styles>
+      <Style Selector="Border.toast">
+        <Setter Property="Background" Value="#323232"/>
+      </Style>
+      <Style Selector="Border.toast.error">
+        <Setter Property="Background" Value="#B3261E"/>
+      </Style>
+    </ItemsControl.Styles>
+    <ItemsControl.ItemTemplate>
+      <DataTemplate x:DataType="svc:ToastItem">
+        <Border Classes="toast" Classes.error="{Binding IsError}"
+                CornerRadius="8" Padding="14,10" MaxWidth="360">
+          <TextBlock Text="{Binding Message}" Foreground="White" FontSize="13" TextWrapping="Wrap"/>
+        </Border>
+      </DataTemplate>
+    </ItemsControl.ItemTemplate>
+  </ItemsControl>
+  </Panel>
 </Window>
diff --git a/LehrerApp.Desktop/Views/Settings/GradingKeyTemplateDialog.axaml b/LehrerApp.Desktop/Views/Settings/GradingKeyTemplateDialog.axaml
index 3db8f18..158ed61 100644
--- a/LehrerApp.Desktop/Views/Settings/GradingKeyTemplateDialog.axaml
+++ b/LehrerApp.Desktop/Views/Settings/GradingKeyTemplateDialog.axaml
@@ -28,15 +28,20 @@
         </ItemsControl.ItemTemplate>
       </ItemsControl>
 
-      <TextBlock Text="{Binding Validation}" Foreground="Red" FontSize="11"
-                 IsVisible="{Binding Validation, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"/>
-
       <Grid ColumnDefinitions="*,8,90,8,Auto">
-        <TextBox Grid.Column="0" Text="{Binding NewGrade}"
-                 PlaceholderText="Note  (z.B. 2 oder 11)" FontSize="12"/>
-        <NumericUpDown Grid.Column="2" Value="{Binding NewMinPercent}" Minimum="0" Maximum="100"
-                       FormatString="0.##" ShowButtonSpinner="False" FontSize="12"
-                       ToolTip.Tip="Mindest-Prozentgrenze"/>
+        <StackPanel Grid.Column="0" Spacing="2">
+          <TextBox Text="{Binding NewGrade}"
+                   PlaceholderText="Note  (z.B. 2 oder 11)" FontSize="12"/>
+          <TextBlock Text="{Binding NewGradeError}" Foreground="Red" FontSize="11"
+                     IsVisible="{Binding NewGradeError, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"/>
+        </StackPanel>
+        <StackPanel Grid.Column="2" Spacing="2">
+          <NumericUpDown Value="{Binding NewMinPercent}" Minimum="0" Maximum="100"
+                         FormatString="0.##" ShowButtonSpinner="False" FontSize="12"
+                         ToolTip.Tip="Mindest-Prozentgrenze"/>
+          <TextBlock Text="{Binding NewMinPercentError}" Foreground="Red" FontSize="11"
+                     IsVisible="{Binding NewMinPercentError, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"/>
+        </StackPanel>
         <Button Grid.Column="4" Content="＋" Command="{Binding AddEntryCommand}"/>
       </Grid>
     </StackPanel>
diff --git a/LehrerApp.Desktop/Views/Settings/SettingsView.axaml b/LehrerApp.Desktop/Views/Settings/SettingsView.axaml
index 0ef11da..bd7ad5b 100644
--- a/LehrerApp.Desktop/Views/Settings/SettingsView.axaml
+++ b/LehrerApp.Desktop/Views/Settings/SettingsView.axaml
@@ -47,8 +47,8 @@
                   <Button  Grid.Column="4" Content="Hinzufügen"
                            Command="{Binding AddSubjectCommand}"/>
                 </Grid>
-                <TextBlock Text="{Binding ValidationMessage}" Foreground="Red" FontSize="12"
-                           IsVisible="{Binding ValidationMessage, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"/>
+                <TextBlock Text="{Binding NewNameError}" Foreground="Red" FontSize="12"
+                           IsVisible="{Binding NewNameError, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"/>
               </StackPanel>
             </Border>
 
@@ -213,8 +213,8 @@
                   <Button  Grid.Column="4" Content="Anlegen"
                            Command="{Binding AddGradingKeyTemplateCommand}"/>
                 </Grid>
-                <TextBlock Text="{Binding TemplateValidationMessage}" Foreground="Red" FontSize="12"
-                           IsVisible="{Binding TemplateValidationMessage, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"/>
+                <TextBlock Text="{Binding NewTemplateNameError}" Foreground="Red" FontSize="12"
+                           IsVisible="{Binding NewTemplateNameError, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"/>
               </StackPanel>
             </Border>
 
diff --git a/LehrerApp.Desktop/Views/Students/AddStudentDialog.axaml b/LehrerApp.Desktop/Views/Students/AddStudentDialog.axaml
index 6679367..66ae3d5 100644
--- a/LehrerApp.Desktop/Views/Students/AddStudentDialog.axaml
+++ b/LehrerApp.Desktop/Views/Students/AddStudentDialog.axaml
@@ -19,10 +19,14 @@
           <StackPanel Grid.Column="0" Spacing="4">
             <TextBlock Text="Nachname *" FontSize="12" Opacity="0.7"/>
             <TextBox Text="{Binding LastName}" PlaceholderText="Mustermann" x:Name="LastNameBox"/>
+            <TextBlock Text="{Binding LastNameError}" Foreground="Red" FontSize="11"
+                       IsVisible="{Binding LastNameError, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"/>
           </StackPanel>
           <StackPanel Grid.Column="2" Spacing="4">
             <TextBlock Text="Vorname *" FontSize="12" Opacity="0.7"/>
             <TextBox Text="{Binding FirstName}" PlaceholderText="Max"/>
+            <TextBlock Text="{Binding FirstNameError}" Foreground="Red" FontSize="11"
+                       IsVisible="{Binding FirstNameError, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"/>
           </StackPanel>
         </Grid>
 
@@ -31,6 +35,8 @@
           <StackPanel Grid.Column="0" Spacing="4">
             <TextBlock Text="Geburtsdatum" FontSize="12" Opacity="0.7"/>
             <TextBox Text="{Binding DateOfBirthText}" PlaceholderText="TT.MM.JJJJ"/>
+            <TextBlock Text="{Binding DateOfBirthError}" Foreground="Red" FontSize="11"
+                       IsVisible="{Binding DateOfBirthError, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"/>
           </StackPanel>
           <StackPanel Grid.Column="2" Spacing="4">
             <TextBlock Text="Geschlecht" FontSize="12" Opacity="0.7"/>
@@ -99,9 +105,6 @@
                    AcceptsReturn="True" Height="70" TextWrapping="Wrap"/>
         </StackPanel>
 
-        <!-- Validierungsfehler -->
-        <TextBlock Text="{Binding ValidationMessage}" Foreground="Red" FontSize="12"
-                   IsVisible="{Binding ValidationMessage, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"/>
       </StackPanel>
     </ScrollViewer>
 
diff --git a/LehrerApp.Desktop/Views/Students/ContactEditDialog.axaml b/LehrerApp.Desktop/Views/Students/ContactEditDialog.axaml
index 8515b5a..c3041d7 100644
--- a/LehrerApp.Desktop/Views/Students/ContactEditDialog.axaml
+++ b/LehrerApp.Desktop/Views/Students/ContactEditDialog.axaml
@@ -17,6 +17,8 @@
           <StackPanel Grid.Column="0" Spacing="4">
             <TextBlock Text="Name *" FontSize="12" Opacity="0.7"/>
             <TextBox x:Name="NameBox" Text="{Binding Name}" PlaceholderText="Name"/>
+            <TextBlock Text="{Binding NameError}" Foreground="Red" FontSize="11"
+                       IsVisible="{Binding NameError, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"/>
           </StackPanel>
           <StackPanel Grid.Column="2" Spacing="4">
             <TextBlock Text="Beziehung" FontSize="12" Opacity="0.7"/>
@@ -56,6 +58,8 @@
             <StackPanel Grid.Column="0" Spacing="4">
               <TextBlock Text="Ungültig seit *" FontSize="12" Opacity="0.7"/>
               <TextBox Text="{Binding InvalidSinceText}" PlaceholderText="TT.MM.JJJJ"/>
+              <TextBlock Text="{Binding InvalidSinceError}" Foreground="Red" FontSize="11"
+                         IsVisible="{Binding InvalidSinceError, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"/>
             </StackPanel>
             <StackPanel Grid.Column="2" Spacing="4">
               <TextBlock Text="Grund" FontSize="12" Opacity="0.7"/>
@@ -72,8 +76,6 @@
           </StackPanel>
         </StackPanel>
 
-        <TextBlock Text="{Binding ValidationMessage}" Foreground="Red" FontSize="12"
-                   IsVisible="{Binding ValidationMessage, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"/>
       </StackPanel>
     </ScrollViewer>
 
diff --git a/LehrerApp.Tests/AppLoggerTests.cs b/LehrerApp.Tests/AppLoggerTests.cs
new file mode 100644
index 0000000..ca0cb07
--- /dev/null
+++ b/LehrerApp.Tests/AppLoggerTests.cs
@@ -0,0 +1,105 @@
+using LehrerApp.Core.Services;
+using Xunit;
+
+namespace LehrerApp.Tests;
+
+public sealed class AppLoggerTests
+{
+    [Fact]
+    public void Konstruktor_LegtLogVerzeichnisAn()
+    {
+        using var temp = new TempAppData();
+
+        _ = new AppLogger(temp.Path);
+
+        Assert.True(Directory.Exists(Path.Combine(temp.Path, "logs")));
+    }
+
+    [Fact]
+    public void Info_SchreibtZeileInDieHeutigeLogdatei()
+    {
+        using var temp = new TempAppData();
+        var logger = new AppLogger(temp.Path);
+
+        logger.Info("Anwendung gestartet.");
+
+        var expectedFile = Path.Combine(temp.Path, "logs", $"lehrerapp-{DateTime.Now:yyyy-MM-dd}.log");
+        Assert.True(File.Exists(expectedFile));
+        var content = File.ReadAllText(expectedFile);
+        Assert.Contains("[INFO ]", content);
+        Assert.Contains("Anwendung gestartet.", content);
+    }
+
+    [Fact]
+    public void Error_MitException_SchreibtStapelverfolgungMit()
+    {
+        using var temp = new TempAppData();
+        var logger = new AppLogger(temp.Path);
+
+        try { throw new InvalidOperationException("Testfehler"); }
+        catch (Exception ex) { logger.Error("Etwas ist schiefgelaufen.", ex); }
+
+        var content = File.ReadAllText(Path.Combine(temp.Path, "logs", $"lehrerapp-{DateTime.Now:yyyy-MM-dd}.log"));
+        Assert.Contains("[ERROR]", content);
+        Assert.Contains("Testfehler", content);
+        Assert.Contains("InvalidOperationException", content);
+    }
+
+    [Fact]
+    public void MehrereEintraege_WerdenAlleAngehaengt()
+    {
+        using var temp = new TempAppData();
+        var logger = new AppLogger(temp.Path);
+
+        logger.Info("Erster Eintrag");
+        logger.Warn("Zweiter Eintrag");
+        logger.Error("Dritter Eintrag");
+
+        var lines = File.ReadAllLines(Path.Combine(temp.Path, "logs", $"lehrerapp-{DateTime.Now:yyyy-MM-dd}.log"));
+        Assert.Equal(3, lines.Length);
+    }
+
+    [Fact]
+    public void Konstruktor_LoeschtLogdateienAelterAlsDieAufbewahrungsfrist()
+    {
+        using var temp = new TempAppData();
+        var logDir = Path.Combine(temp.Path, "logs");
+        Directory.CreateDirectory(logDir);
+
+        var oldFile = Path.Combine(logDir, "lehrerapp-2000-01-01.log");
+        var recentFile = Path.Combine(logDir, $"lehrerapp-{DateTime.Now:yyyy-MM-dd}.log");
+        File.WriteAllText(oldFile, "alt");
+        File.WriteAllText(recentFile, "aktuell");
+        File.SetLastWriteTime(oldFile, DateTime.Now.AddDays(-30));
+
+        _ = new AppLogger(temp.Path);
+
+        Assert.False(File.Exists(oldFile));
+        Assert.True(File.Exists(recentFile));
+    }
+
+    [Fact]
+    public void Konstruktor_BehaeltLogdateienInnerhalbDerAufbewahrungsfrist()
+    {
+        using var temp = new TempAppData();
+        var logDir = Path.Combine(temp.Path, "logs");
+        Directory.CreateDirectory(logDir);
+
+        var recentButOldFile = Path.Combine(logDir, "lehrerapp-recent.log");
+        File.WriteAllText(recentButOldFile, "vor 5 Tagen");
+        File.SetLastWriteTime(recentButOldFile, DateTime.Now.AddDays(-5));
+
+        _ = new AppLogger(temp.Path);
+
+        Assert.True(File.Exists(recentButOldFile));
+    }
+
+    private sealed class TempAppData : IDisposable
+    {
+        public string Path { get; } = System.IO.Path.Combine(
+            System.IO.Path.GetTempPath(), $"lehrerapp-logger-tests-{Guid.NewGuid():N}");
+
+        public TempAppData() => Directory.CreateDirectory(Path);
+        public void Dispose() { if (Directory.Exists(Path)) Directory.Delete(Path, recursive: true); }
+    }
+}
diff --git a/TODO.md b/TODO.md
index bc9eff3..80192d7 100644
--- a/TODO.md
+++ b/TODO.md
@@ -491,10 +491,43 @@ Fächer- und Kompetenzverwaltung existiert bereits in
       `.ToUniversalTime()` auf beiden Seiten vor dem Vergleich.
 
 ### 13.2 Fehlerbehandlung & Logging
-- [ ] **13.2.1** Zentrale Exception-Behandlung mit verständlicher Fehlermeldung statt Absturz.
-- [ ] **13.2.2** Logging in Datei im App-Datenverzeichnis, mit Rotation.
-- [ ] **13.2.3** Einheitliche Benachrichtigungen (Toast/Snackbar) für Erfolg und Fehler.
-- [ ] **13.2.4** Validierungsmeldungen einheitlich an den Eingabefeldern statt in Sammel-Labels.
+- [x] **13.2.1** Zentrale Exception-Behandlung mit verständlicher Fehlermeldung statt Absturz —
+      [GlobalExceptionHandler.cs](LehrerApp.Desktop/Services/GlobalExceptionHandler.cs).
+      `Dispatcher.UIThread.UnhandledException` fängt Fehler aus Befehlen/Ereignis-Handlern auf
+      dem UI-Thread ab, protokolliert sie und setzt `Handled = true` — die App stürzt dabei
+      nachweislich nicht ab (per Headless-Test verifiziert: Fehler in einem Dispatcher-Callback
+      wird geloggt + als Toast angezeigt, App läuft weiter). `AppDomain.UnhandledException` und
+      `TaskScheduler.UnobservedTaskException` sind Sicherheitsnetze für Fehler außerhalb des
+      UI-Threads — dort kann ein bereits "IsTerminating"-Fehler nicht mehr verhindert werden,
+      wird aber vollständig protokolliert.
+- [x] **13.2.2** Logging in Datei im App-Datenverzeichnis, mit Rotation —
+      [AppLogger.cs](LehrerApp.Core/Services/AppLogger.cs), tägliche Datei unter
+      `<AppData>/LehrerApp/logs/`, Dateien älter als 14 Tage werden beim Start gelöscht.
+      6 Tests in [AppLoggerTests.cs](LehrerApp.Tests/AppLoggerTests.cs).
+- [x] **13.2.3** Einheitliche Benachrichtigungen (Toast/Snackbar) für Erfolg und Fehler —
+      [NotificationService.cs](LehrerApp.Desktop/Services/NotificationService.cs), Overlay unten
+      rechts in [MainWindow.axaml](LehrerApp.Desktop/Views/MainWindow.axaml), blendet sich nach
+      3 s (Erfolg) bzw. 6 s (Fehler) automatisch aus. Aktuell an den globalen Exception-Handler
+      angebunden; bestehende Dialoge zeigen Erfolg/Fehler weiterhin über ihre eigenen
+      Statuszeilen (`StatusMessage`/`ValidationMessage`) — eine Umstellung dieser bestehenden
+      Stellen auf Toasts wäre ein eigener, größerer Umbau über viele Dateien hinweg und ist
+      bewusst nicht Teil dieser Aufgabe.
+- [x] **13.2.4** Validierungsmeldungen einheitlich an den Eingabefeldern statt in Sammel-Labels.
+      Alle Dialoge mit Formularfeldern umgestellt: `AddStudentDialog`, `ContactEditDialog`,
+      `AddGroupDialog`, `StudentGradesDialog` (`GradeEditItem`), `CollectiveGradeDialog`,
+      `ExamDialog`, `AddSessionDialog`, `ParticipationWizardDialog` (Abschnitt-Formular),
+      Einstellungen (Fach anlegen, Notenschlüssel-Vorlage anlegen, Notenschlüssel-Editor).
+      Muster: pro Feld eine `{Feld}Error`-Property, `Save()` leert alle Fehler-Properties am
+      Anfang, sammelt alle Verstöße über eine lokale `valid`-Variable statt beim ersten Fehler
+      zurückzukehren, und bricht erst am Ende mit `if (!valid) return;` ab — so werden mehrere
+      Fehler gleichzeitig angezeigt. Bewusst als Sammel-Meldung belassen, wo die Meldung sich auf
+      eine Kombination mehrerer Felder bezieht statt auf ein einzelnes Feld: `GradingKeyValidation`
+      (Notenschlüssel-Tabelle als Ganzes), `GradingSchemeEditItem.ValidationMessage`
+      (Summe der drei Prozent-Felder muss 100 ergeben), `ReportGradeRow.ValidationMessage`
+      (Override-Wert + Begründung gehören zusammen), `CatalogValidation`
+      (Kompetenzkatalog-Panel: Fachauswahl, Bereichsname und JSON-Import teilen sich einen
+      Status-Bereich), `AddStudentToGroupDialogViewModel.ValidationMessage` (einzige Prüfung
+      "Schüler ausgewählt?", kein Textfeld zum Andocken vorhanden).
 
 ### 13.3 Datensicherheit
 - [ ] **13.3.1** Automatisches lokales Backup der LiteDB beim Start (rollierend, letzte 10).