diff --git a/Services/ICalendarService.cs b/Services/ICalendarService.cs
new file mode 100644
index 0000000..d5ce3ce
--- /dev/null
+++ b/Services/ICalendarService.cs
@@ -0,0 +1,15 @@
+using ShiftScheduler.Shared;
+
+namespace ShiftScheduler.Services;
+
+public interface ICalendarService
+{
+    Task<List<CalendarInfo>> GetCalendarsAsync();
+    Task SyncShiftsToCalendarAsync(string calendarId, List<ShiftWithTransport> shifts);
+}
+
+/// <summary>Marker interface for DI disambiguation.</summary>
+public interface IGoogleCalendarService : ICalendarService { }
+
+/// <summary>Marker interface for DI disambiguation.</summary>
+public interface INextcloudCalendarService : ICalendarService { }