diff --git a/LehrerApp.Desktop/Services/WebUntisIntegrationService.cs b/LehrerApp.Desktop/Services/WebUntisIntegrationService.cs
index 7d0c3e0..1ed0f5f 100644
--- a/LehrerApp.Desktop/Services/WebUntisIntegrationService.cs
+++ b/LehrerApp.Desktop/Services/WebUntisIntegrationService.cs
@@ -205,6 +205,13 @@ public sealed class WebUntisIntegrationService(HttpClient http, WebUntisSettings
         };
     }
 
+    public Task<UntisOpenPeriodsMeta> GetOpenPeriodsMetaAsync(int schoolYearId, CancellationToken token = default) =>
+        ExecuteAsync(client => client.GetOpenPeriodsMetaAsync(schoolYearId, token), token);
+
+    public Task<IReadOnlyList<UntisOpenPeriod>> GetOpenPeriodsAsync(int schoolYearId, int? teacherId,
+        int? classId, DateOnly start, DateOnly end, CancellationToken token = default) =>
+        ExecuteAsync(client => client.GetOpenPeriodsAsync(schoolYearId, teacherId, classId, start, end, token), token);
+
     private async Task<T> ExecuteAsync<T>(Func<WebUntisClient, Task<T>> operation, CancellationToken token)
     {
         try { return await operation(await GetClientAsync(token)); }
diff --git a/LehrerApp.Desktop/Views/MainWindow.axaml b/LehrerApp.Desktop/Views/MainWindow.axaml
index 76e9213..7a5f8ee 100644
--- a/LehrerApp.Desktop/Views/MainWindow.axaml
+++ b/LehrerApp.Desktop/Views/MainWindow.axaml
@@ -250,6 +250,8 @@
                 <TextBlock Classes="navlabel" Text="Klassenlehrer"/>
               </StackPanel>
             </Button>
+            <Button Classes="navitem" HorizontalAlignment="Stretch"
+                    Content="Offene Untis-Stunden" Click="OnOpenUntisPeriods"/>
             <Button Classes="navitem" Classes.active="{Binding IsSettingsActive}" HorizontalAlignment="Stretch"
                     HorizontalContentAlignment="Left"
                     CornerRadius="6"
diff --git a/LehrerApp.Desktop/Views/MainWindow.axaml.cs b/LehrerApp.Desktop/Views/MainWindow.axaml.cs
index 717bbd9..6dd06f4 100644
--- a/LehrerApp.Desktop/Views/MainWindow.axaml.cs
+++ b/LehrerApp.Desktop/Views/MainWindow.axaml.cs
@@ -23,6 +23,11 @@ public partial class MainWindow : Window
         KeyDown        += OnWindowKeyDown;
     }
 
+    private async void OnOpenUntisPeriods(object? sender, Avalonia.Interactivity.RoutedEventArgs e)
+    {
+        await new OpenUntisPeriodsDialog().ShowDialog(this);
+    }
+
     private void OnWindowKeyDown(object? sender, KeyEventArgs e)
     {
         if (DataContext is not MainWindowViewModel vm) return;
diff --git a/LehrerApp.Desktop/Views/OpenUntisPeriodsDialog.cs b/LehrerApp.Desktop/Views/OpenUntisPeriodsDialog.cs
new file mode 100644
index 0000000..ffe0832
--- /dev/null
+++ b/LehrerApp.Desktop/Views/OpenUntisPeriodsDialog.cs
@@ -0,0 +1,125 @@
+using Avalonia;
+using Avalonia.Controls;
+using LehrerApp.Desktop.Services;
+using LehrerApp.WebUntis;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace LehrerApp.Desktop.Views;
+
+public sealed class OpenUntisPeriodsDialog : Window
+{
+    private readonly WebUntisIntegrationService _service = App.Services.GetRequiredService<WebUntisIntegrationService>();
+    private readonly WebUntisSettingsService _settings = App.Services.GetRequiredService<WebUntisSettingsService>();
+    private readonly CancellationTokenSource _closed = new();
+    private readonly ComboBox _mode = new() { ItemsSource = new[] { "Mein Unterricht", "Meine Klasse" }, SelectedIndex = 0 };
+    private readonly ComboBox _element = new() { MinWidth = 150 };
+    private readonly DatePicker _start = new();
+    private readonly DatePicker _end = new() { SelectedDate = DateTimeOffset.Now };
+    private readonly Button _load = new() { Content = "Abrufen" };
+    private readonly Button _schoolYear = new() { Content = "Seit Schuljahresbeginn" };
+    private readonly TextBlock _status = new() { TextWrapping = Avalonia.Media.TextWrapping.Wrap };
+    private readonly StackPanel _rows = new() { Spacing = 12 };
+    private readonly StackPanel _filters = new() { Spacing = 10, IsEnabled = false };
+    private UntisOpenPeriodsMeta? _meta;
+    private int _yearId;
+
+    public OpenUntisPeriodsDialog()
+    {
+        Title = "Offene WebUntis-Stunden";
+        Width = 850; Height = 650; MinWidth = 600; MinHeight = 450;
+        WindowStartupLocation = WindowStartupLocation.CenterOwner;
+        var root = new DockPanel { Margin = new Thickness(20) };
+        var header = new StackPanel { Spacing = 12 };
+        header.Children.Add(new TextBlock { Text = Title, FontSize = 22 });
+        var selection = new StackPanel { Orientation = Avalonia.Layout.Orientation.Horizontal, Spacing = 12 };
+        selection.Children.Add(_mode); selection.Children.Add(_element);
+        _filters.Children.Add(selection);
+        var dates = new WrapPanel { Orientation = Avalonia.Layout.Orientation.Horizontal };
+        dates.Children.Add(new TextBlock { Text = "Von ", VerticalAlignment = Avalonia.Layout.VerticalAlignment.Center });
+        dates.Children.Add(_start);
+        dates.Children.Add(new TextBlock { Text = "  bis ", VerticalAlignment = Avalonia.Layout.VerticalAlignment.Center });
+        dates.Children.Add(_end);
+        _filters.Children.Add(dates);
+        var actions = new StackPanel { Orientation = Avalonia.Layout.Orientation.Horizontal, Spacing = 12 };
+        actions.Children.Add(_schoolYear); actions.Children.Add(_load);
+        _filters.Children.Add(actions);
+        header.Children.Add(_filters); header.Children.Add(_status);
+        DockPanel.SetDock(header, Dock.Top); root.Children.Add(header);
+        root.Children.Add(new ScrollViewer { Content = _rows, Margin = new Thickness(0, 20, 0, 0) });
+        Content = root;
+        _mode.SelectionChanged += (_, _) => SelectMode();
+        _element.SelectionChanged += (_, _) => ClearResults();
+        _start.SelectedDateChanged += (_, _) => ClearResults();
+        _end.SelectedDateChanged += (_, _) => ClearResults();
+        _schoolYear.Click += (_, _) => { if (_meta is not null) _start.SelectedDate = new DateTimeOffset(_meta.SchoolYearStart.ToDateTime(TimeOnly.MinValue)); };
+        _load.Click += async (_, _) => await LoadAsync();
+        Opened += async (_, _) => await InitializeAsync();
+        Closed += (_, _) => _closed.Cancel();
+    }
+
+    private void ClearResults() { _rows.Children.Clear(); _status.Text = "Auswahl festlegen und Abrufen wählen."; }
+
+    private void SelectMode()
+    {
+        if (_meta is null) return;
+        var own = _mode.SelectedIndex == 0;
+        var elements = own ? _meta.Teachers : _meta.Classes;
+        _element.ItemsSource = elements;
+        var preferred = own ? _settings.TeacherUntisId ?? _meta.OwnTeacherId
+            : _settings.HomeroomClassUntisId ?? _meta.MyClassIds.Cast<int?>().FirstOrDefault();
+        _element.SelectedItem = elements.FirstOrDefault(x => x.Id == preferred)
+            ?? (!own ? elements.FirstOrDefault(x => _meta.MyClassIds.Contains(x.Id)) : null);
+        ClearResults();
+    }
+
+    private async Task InitializeAsync()
+    {
+        _status.Text = "WebUntis-Auswahl wird geladen …";
+        try
+        {
+            var today = DateOnly.FromDateTime(DateTime.Today);
+            var date = today.Year * 10000 + today.Month * 100 + today.Day;
+            var years = await _service.GetSchoolYearsAsync(_closed.Token);
+            var year = years.FirstOrDefault(x => x.StartDate <= date && date <= x.EndDate)
+                ?? throw new InvalidDataException("Kein aktuelles WebUntis-Schuljahr gefunden.");
+            _yearId = year.UntisId;
+            _meta = await _service.GetOpenPeriodsMetaAsync(_yearId, _closed.Token);
+            _start.SelectedDate = new DateTimeOffset(_meta.SchoolYearStart.ToDateTime(TimeOnly.MinValue));
+            SelectMode();
+            _filters.IsEnabled = true;
+            if (_element.SelectedItem is not null) await LoadAsync();
+        }
+        catch (OperationCanceledException) when (_closed.IsCancellationRequested) { }
+        catch (Exception ex) { _status.Text = $"Abruf fehlgeschlagen: {ex.Message}"; }
+    }
+
+    private async Task LoadAsync()
+    {
+        _rows.Children.Clear();
+        if (_element.SelectedItem is not UntisOpenElement element || _start.SelectedDate is null || _end.SelectedDate is null)
+        { _status.Text = "Bitte Lehrkraft oder Klasse und Zeitraum auswählen."; return; }
+        var start = DateOnly.FromDateTime(_start.SelectedDate.Value.DateTime);
+        var end = DateOnly.FromDateTime(_end.SelectedDate.Value.DateTime);
+        if (start > end || start < _meta!.SchoolYearStart || end > _meta.SchoolYearEnd)
+        { _status.Text = "Bitte einen gültigen Zeitraum innerhalb des Schuljahres auswählen."; return; }
+        _filters.IsEnabled = false;
+        _status.Text = "Offene Stunden werden geladen …";
+        try
+        {
+            var own = _mode.SelectedIndex == 0;
+            var periods = await _service.GetOpenPeriodsAsync(_yearId, own ? element.Id : null,
+                own ? null : element.Id, start, end, _closed.Token);
+            foreach (var period in periods)
+                _rows.Children.Add(new TextBlock
+                {
+                    Text = $"{period.When}\n{period.Classes} · {period.Subject} · {period.Teachers}\n{period.Missing}",
+                    TextWrapping = Avalonia.Media.TextWrapping.Wrap
+                });
+            _status.Text = periods.Count == 0 ? "Keine offenen Stunden im gewählten Zeitraum."
+                : $"{periods.Count} offene Stunde(n) · Stand {DateTime.Now:HH:mm}";
+        }
+        catch (OperationCanceledException) when (_closed.IsCancellationRequested) { }
+        catch (Exception ex) { _status.Text = $"Abruf fehlgeschlagen: {ex.Message}"; }
+        finally { _filters.IsEnabled = true; }
+    }
+}
diff --git a/LehrerApp.WebUntis.Tests/WebUntisOpenPeriodsTests.cs b/LehrerApp.WebUntis.Tests/WebUntisOpenPeriodsTests.cs
new file mode 100644
index 0000000..b74c81a
--- /dev/null
+++ b/LehrerApp.WebUntis.Tests/WebUntisOpenPeriodsTests.cs
@@ -0,0 +1,95 @@
+using System.Net;
+using System.Text.Json;
+using LehrerApp.WebUntis;
+using Xunit;
+
+namespace LehrerApp.WebUntis.Tests;
+
+public sealed class WebUntisOpenPeriodsTests
+{
+    [Theory]
+    [InlineData(true, false, true, "", true, false)]
+    [InlineData(false, true, true, "Bruchrechnung", false, true)]
+    [InlineData(false, true, true, "  ", true, true)]
+    [InlineData(false, false, false, "", false, false)]
+    public void MissingFlagsRespectRequirementsAndIgnoreTopicId(bool checkedAbs, bool neededAbs,
+        bool neededTopic, string topic, bool missingTopic, bool missingAbs)
+    {
+        var json = JsonSerializer.SerializeToElement(new
+        {
+            periods = new[] { new {
+                period = new { id = 123, hr = 2,
+                    dtRange = new { start = "2026-09-08T08:35:00", end = "2026-09-08T09:20:00" },
+                    classes = new[] { new { el = new { id = 1, nameShort = "7a" } } },
+                    teachers = new[] { new { el = new { id = 2, nameShort = "ABC" } } },
+                    subject = new { el = new { name = "Mat" } } },
+                absChecked = checkedAbs, absCheckNeeded = neededAbs, topicNeeded = neededTopic,
+                topicShort = topic, topicId = 999 } }, total = 80
+        });
+        var row = Assert.Single(WebUntisClient.ParseOpenPeriods(json));
+        Assert.Equal(missingTopic, row.TopicMissing);
+        Assert.Equal(missingAbs, row.AttendanceMissing);
+        Assert.Equal(DateTimeKind.Unspecified, row.Start.Kind);
+        Assert.Equal("7a", row.Classes);
+    }
+
+    [Theory]
+    [InlineData(true)]
+    [InlineData(false)]
+    public async Task RequestUsesExclusiveFilterAndSessionToken(bool teacher)
+    {
+        var handler = new Handler();
+        await using var client = new WebUntisClient(new HttpClient(handler), new WebUntisOptions
+        { School = "test", Host = "test.webuntis.com", Username = "test", Password = "test" });
+        var meta = await client.GetOpenPeriodsMetaAsync(22, CancellationToken.None);
+        Assert.Equal(2, meta.OwnTeacherId);
+        Assert.Equal(1, Assert.Single(meta.MyClassIds));
+        var rows = await client.GetOpenPeriodsAsync(22, teacher ? 2 : null, teacher ? null : 1,
+            new DateOnly(2026, 8, 13), new DateOnly(2026, 9, 8), CancellationToken.None);
+        Assert.Empty(rows);
+        using var body = JsonDocument.Parse(handler.Body!);
+        Assert.Equal(teacher ? 2 : 1, body.RootElement.GetProperty(teacher ? "teacherId" : "classId").GetInt32());
+        Assert.False(body.RootElement.TryGetProperty(teacher ? "classId" : "teacherId", out _));
+        Assert.Equal("TOPIC_OR_ABSENCE_OPEN", body.RootElement.GetProperty("filter").GetString());
+        Assert.Equal("2026-08-13", body.RootElement.GetProperty("dateRange").GetProperty("start").GetString());
+    }
+
+    private sealed class Handler : HttpMessageHandler
+    {
+        public string? Body { get; private set; }
+        protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken token)
+        {
+            var path = request.RequestUri!.AbsolutePath;
+            string response;
+            if (path.EndsWith("jsonrpc.do"))
+                response = "{\"result\":{\"sessionId\":\"test-session\",\"personId\":2}}";
+            else if (path.EndsWith("token/new"))
+            {
+                Assert.Contains("test-session", request.Headers.GetValues("Cookie").Single());
+                response = "test.token.value";
+            }
+            else
+            {
+                Assert.Equal("Bearer", request.Headers.Authorization!.Scheme);
+                Assert.Equal("test.token.value", request.Headers.Authorization.Parameter);
+                Assert.Equal("22", request.Headers.GetValues("x-webuntis-api-school-year-id").Single());
+                if (path.EndsWith("/meta"))
+                {
+                    Assert.Equal(HttpMethod.Get, request.Method);
+                    response = """
+                        {"teachers":[{"el":{"id":2,"nameShort":"ABC"}}],
+                        "classes":[{"el":{"id":1,"nameShort":"7a"}}],"myClassIds":[1],
+                        "schoolYear":{"start":"2026-08-13","end":"2027-07-07"}}
+                        """;
+                }
+                else
+                {
+                    Assert.Equal(HttpMethod.Post, request.Method);
+                    Body = await request.Content!.ReadAsStringAsync(token);
+                    response = "{\"periods\":[],\"total\":80}";
+                }
+            }
+            return new HttpResponseMessage(HttpStatusCode.OK) { Content = new StringContent(response) };
+        }
+    }
+}
diff --git a/LehrerApp.WebUntis/WebUntisClient.cs b/LehrerApp.WebUntis/WebUntisClient.cs
index 70e71cf..1e4b3b3 100644
--- a/LehrerApp.WebUntis/WebUntisClient.cs
+++ b/LehrerApp.WebUntis/WebUntisClient.cs
@@ -5,7 +5,7 @@ using System.Text.Json;
 
 namespace LehrerApp.WebUntis;
 
-public sealed class WebUntisClient : IAsyncDisposable
+public sealed partial class WebUntisClient : IAsyncDisposable
 {
     private static readonly JsonSerializerOptions JsonOptions = new(JsonSerializerDefaults.Web);
     private static readonly UTF8Encoding StrictUtf8 = new(false, true);
diff --git a/LehrerApp.WebUntis/WebUntisOpenPeriods.cs b/LehrerApp.WebUntis/WebUntisOpenPeriods.cs
new file mode 100644
index 0000000..d03724f
--- /dev/null
+++ b/LehrerApp.WebUntis/WebUntisOpenPeriods.cs
@@ -0,0 +1,113 @@
+using System.Net.Http.Headers;
+using System.Net.Http.Json;
+using System.Text.Json;
+
+namespace LehrerApp.WebUntis;
+
+public sealed record UntisOpenElement(int Id, string Name)
+{
+    public override string ToString() => Name;
+}
+public sealed record UntisOpenPeriodsMeta(
+    IReadOnlyList<UntisOpenElement> Teachers, IReadOnlyList<UntisOpenElement> Classes,
+    IReadOnlyList<int> MyClassIds, DateOnly SchoolYearStart, DateOnly SchoolYearEnd,
+    int? OwnTeacherId);
+public sealed record UntisOpenPeriod(int Id, DateTime Start, DateTime End, int Hour,
+    string Classes, string Teachers, string Subject, bool TopicMissing, bool AttendanceMissing)
+{
+    public string When => $"{Start:dd.MM.yyyy} · {Hour}. Stunde · {Start:HH:mm}–{End:HH:mm}";
+    public string Missing => (TopicMissing, AttendanceMissing) switch
+    {
+        (true, true) => "Thema und Anwesenheitskontrolle fehlen",
+        (true, false) => "Thema fehlt",
+        (false, true) => "Anwesenheitskontrolle fehlt",
+        _ => "Von WebUntis als offen gemeldet"
+    };
+}
+
+public sealed partial class WebUntisClient
+{
+    private const string OpenPeriodsPath = "/WebUntis/api/rest/view/v1/classreg/open-periods";
+
+    public Task<UntisOpenPeriodsMeta> GetOpenPeriodsMetaAsync(int schoolYearId, CancellationToken token) =>
+        WithSessionAsync(async session =>
+        {
+            var json = await OpenPeriodsRequestAsync(session, schoolYearId, null, token);
+            var year = json.GetProperty("schoolYear");
+            return new UntisOpenPeriodsMeta(OpenElements(json.GetProperty("teachers")),
+                OpenElements(json.GetProperty("classes")),
+                json.GetProperty("myClassIds").EnumerateArray().Select(x => x.GetInt32()).ToArray(),
+                year.GetProperty("start").GetDateOnly(), year.GetProperty("end").GetDateOnly(),
+                _myTeacherUntisId);
+        }, token);
+
+    public Task<IReadOnlyList<UntisOpenPeriod>> GetOpenPeriodsAsync(int schoolYearId, int? teacherId,
+        int? classId, DateOnly start, DateOnly end, CancellationToken token)
+    {
+        if ((teacherId is null) == (classId is null) || teacherId is <= 0 || classId is <= 0)
+            throw new ArgumentException("Genau eine gültige Lehrer- oder Klassen-ID ist erforderlich.");
+        if (start > end) throw new ArgumentException("Der Beginn muss vor dem Ende liegen.");
+        var body = new Dictionary<string, object>
+        {
+            [teacherId.HasValue ? "teacherId" : "classId"] = teacherId ?? classId!.Value,
+            ["filter"] = "TOPIC_OR_ABSENCE_OPEN",
+            ["dateRange"] = new { start = start.ToString("yyyy-MM-dd"), end = end.ToString("yyyy-MM-dd") }
+        };
+        return WithSessionAsync(async session => ParseOpenPeriods(
+            await OpenPeriodsRequestAsync(session, schoolYearId, body, token)), token);
+    }
+
+    public static IReadOnlyList<UntisOpenPeriod> ParseOpenPeriods(JsonElement json) =>
+        json.GetProperty("periods").EnumerateArray().Select(row =>
+        {
+            var p = row.GetProperty("period");
+            var range = p.GetProperty("dtRange");
+            return new UntisOpenPeriod(p.GetProperty("id").GetInt32(),
+                range.GetProperty("start").GetDateTime(), range.GetProperty("end").GetDateTime(),
+                p.GetProperty("hr").GetInt32(),
+                string.Join(", ", OpenElements(p.GetProperty("classes")).Select(x => x.Name)),
+                string.Join(", ", OpenElements(p.GetProperty("teachers")).Select(x => x.Name)),
+                p.GetProperty("subject").GetProperty("el").GetProperty("name").GetString() ?? "",
+                row.GetProperty("topicNeeded").GetBoolean() &&
+                    string.IsNullOrWhiteSpace(row.GetProperty("topicShort").GetString()),
+                row.GetProperty("absCheckNeeded").GetBoolean() && !row.GetProperty("absChecked").GetBoolean());
+        }).OrderBy(x => x.Start).ToArray();
+
+    private static IReadOnlyList<UntisOpenElement> OpenElements(JsonElement array) => array.EnumerateArray()
+        .Select(x => x.GetProperty("el"))
+        .Select(x => new UntisOpenElement(x.GetProperty("id").GetInt32(),
+            x.GetProperty("nameShort").GetString() ?? x.GetProperty("name").GetString() ?? ""))
+        .ToArray();
+
+    private async Task<JsonElement> OpenPeriodsRequestAsync(string session, int schoolYearId,
+        object? body, CancellationToken token)
+    {
+        // The REST UI uses a bearer token issued for the existing WebUntis session.
+        using var tokenRequest = ReportRequest($"https://{GetConfiguration().Host}/WebUntis/api/token/new", session, true);
+        using var tokenResponse = await SendAsync(tokenRequest, TimeSpan.FromSeconds(20), token);
+        if (!tokenResponse.IsSuccessStatusCode)
+            throw new WebUntisException($"WebUntis-Token konnte nicht abgerufen werden: HTTP {(int)tokenResponse.StatusCode}.");
+        var bearer = (await tokenResponse.Content.ReadAsStringAsync(token)).Trim();
+        if (bearer.StartsWith('"')) bearer = JsonSerializer.Deserialize<string>(bearer) ?? "";
+        if (string.IsNullOrWhiteSpace(bearer) ||
+            bearer.Any(c => !char.IsAsciiLetterOrDigit(c) && c is not '-' and not '_' and not '.'))
+            throw new WebUntisException("WebUntis hat keinen gültigen Zugriffstoken geliefert.");
+        using var request = ReportRequest($"https://{GetConfiguration().Host}{OpenPeriodsPath}{(body is null ? "/meta" : "")}", session, true);
+        request.Headers.Authorization = new AuthenticationHeaderValue("Bearer", bearer);
+        request.Headers.Add("x-webuntis-api-school-year-id", schoolYearId.ToString());
+        if (body is not null)
+        {
+            request.Method = HttpMethod.Post;
+            request.Content = JsonContent.Create(body, options: JsonOptions);
+        }
+        using var response = await SendAsync(request, TimeSpan.FromSeconds(30), token);
+        if (!response.IsSuccessStatusCode)
+            throw new WebUntisException($"Offene Stunden konnten nicht abgerufen werden: HTTP {(int)response.StatusCode}.");
+        return await ReadJsonAsync(response, token);
+    }
+}
+
+internal static class OpenPeriodJsonDates
+{
+    public static DateOnly GetDateOnly(this JsonElement value) => DateOnly.ParseExact(value.GetString()!, "yyyy-MM-dd");
+}