diff --git a/tests/__init__.py b/tests/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/tests/conftest.py b/tests/conftest.py
new file mode 100644
index 0000000..22537ff
--- /dev/null
+++ b/tests/conftest.py
@@ -0,0 +1,40 @@
+"""Fixtures for testing caldav2google."""
+
+from unittest.mock import MagicMock
+
+import pytest
+
+
+@pytest.fixture
+def mock_google_service():
+    """Create a mock Google Calendar service."""
+    service = MagicMock()
+    events = MagicMock()
+    service.events.return_value = events
+    return service
+
+
+@pytest.fixture
+def mock_caldav_principal():
+    """Create a mock CalDAV principal."""
+    return MagicMock()
+
+
+@pytest.fixture
+def sample_event_data():
+    """Create sample event data for testing."""
+    return {
+        "test-uid-1": {
+            "uid": "test-uid-1",
+            "summary": "Test Event 1",
+            "description": "Test Description",
+            "location": "Test Location",
+            "start": "2024-01-01T10:00:00+00:00",
+            "end": "2024-01-01T11:00:00+00:00",
+            "last_modified": "2024-01-01T09:00:00+00:00",
+            "rrule": None,
+            "exdate": None,
+            "recurrence_id": None,
+            "google_event_id": "google-event-1",
+        },
+    }
diff --git a/tests/test_auth_google.py b/tests/test_auth_google.py
new file mode 100644
index 0000000..0ccfe80
--- /dev/null
+++ b/tests/test_auth_google.py
@@ -0,0 +1,36 @@
+"""Tests for the auth_google module."""
+
+import pytest
+
+from src.auth_google import search_calendar_id
+
+
+def test_search_calendar_id_found(mock_google_service):
+    """Test searching for a calendar ID that exists in the list of calendars."""
+    calendar_name = "Test Calendar"
+    calendars = {
+        "items": [
+            {"summary": "Wrong Calendar", "id": "wrong-id"},
+            {"summary": "Test Calendar", "id": "correct-id"},
+        ],
+    }
+    mock_google_service.calendarList().list().execute.return_value = calendars
+
+    result = search_calendar_id(mock_google_service, calendar_name)
+
+    assert result == "correct-id"
+
+
+def test_search_calendar_id_not_found(mock_google_service):
+    """Test searching for a calendar ID that does not exist in the list of calendars."""
+    calendar_name = "Nonexistent Calendar"
+    calendars = {
+        "items": [
+            {"summary": "Wrong Calendar", "id": "wrong-id"},
+            {"summary": "Test Calendar", "id": "correct-id"},
+        ],
+    }
+    mock_google_service.calendarList().list().execute.return_value = calendars
+
+    with pytest.raises(ValueError, match=f"No calendar named '{calendar_name}' found"):
+        search_calendar_id(mock_google_service, calendar_name)
diff --git a/tests/test_caldav_client.py b/tests/test_caldav_client.py
new file mode 100644
index 0000000..6b74a4c
--- /dev/null
+++ b/tests/test_caldav_client.py
@@ -0,0 +1,30 @@
+"""Tests for the caldav_client module."""
+
+from unittest.mock import MagicMock
+
+import pytest
+
+from src.caldav_client import get_calendar
+
+
+def test_get_calendar_found(mock_caldav_principal):
+    """Test getting a calendar that exists in the list of calendars."""
+    calendar_name = "Test Calendar"
+    mock_calendar = MagicMock()
+    mock_calendar.name = calendar_name
+    mock_caldav_principal.calendars.return_value = [mock_calendar]
+
+    result = get_calendar(mock_caldav_principal, calendar_name)
+
+    assert result == mock_calendar
+
+
+def test_get_calendar_not_found(mock_caldav_principal):
+    """Test getting a calendar that does not exist in the list of calendars."""
+    calendar_name = "Nonexistent Calendar"
+    mock_calendar = MagicMock()
+    mock_calendar.name = "Different Calendar"
+    mock_caldav_principal.calendars.return_value = [mock_calendar]
+
+    with pytest.raises(ValueError, match=f"No calendar named '{calendar_name}' found"):
+        get_calendar(mock_caldav_principal, calendar_name)
diff --git a/tests/test_sync_logic.py b/tests/test_sync_logic.py
new file mode 100644
index 0000000..498096a
--- /dev/null
+++ b/tests/test_sync_logic.py
@@ -0,0 +1,69 @@
+"""Tests for the sync_logic module."""
+
+from datetime import datetime, timezone
+
+from src.sync_logic import _sanitize_event_for_json, compare_events
+
+
+def test_compare_events_new(sample_event_data):
+    """Test comparing events with a new event."""
+    local_events = {}
+    server_events = sample_event_data
+
+    new_events, updated_events, deleted_events = compare_events(local_events, server_events)
+
+    assert len(new_events) == 1
+    assert len(updated_events) == 0
+    assert len(deleted_events) == 0
+    assert new_events[0]["uid"] == "test-uid-1"
+
+
+def test_compare_events_updated(sample_event_data):
+    """Test comparing events with an updated event."""
+    local_events = {
+        "test-uid-1": {
+            **sample_event_data["test-uid-1"],
+            "last_modified": "2024-01-01T08:00:00+00:00",  # Earlier modification time
+        },
+    }
+    server_events = sample_event_data
+
+    new_events, updated_events, deleted_events = compare_events(local_events, server_events)
+
+    assert len(new_events) == 0
+    assert len(updated_events) == 1
+    assert len(deleted_events) == 0
+    assert updated_events[0]["uid"] == "test-uid-1"
+
+
+def test_compare_events_deleted(sample_event_data):
+    """Test comparing events with a deleted event."""
+    local_events = sample_event_data
+    server_events = {}
+
+    new_events, updated_events, deleted_events = compare_events(local_events, server_events)
+
+    assert len(new_events) == 0
+    assert len(updated_events) == 0
+    assert len(deleted_events) == 1
+    assert deleted_events[0]["uid"] == "test-uid-1"
+
+
+def test_sanitize_event_for_json(sample_event_data):
+    """Test sanitizing an event for JSON serialization."""
+    test_datetime = datetime(2024, 12, 31, tzinfo=timezone.utc)
+
+    event = sample_event_data["test-uid-1"].copy()
+    event["rrule"] = {
+        "FREQ": "WEEKLY",
+        "COUNT": 4,
+        "UNTIL": test_datetime,
+        "BYDAY": ["MO", "WE", "FR"],
+    }
+
+    sanitized = _sanitize_event_for_json(event)
+
+    assert sanitized["rrule"]["FREQ"] == "WEEKLY"
+    assert sanitized["rrule"]["COUNT"] == 4  # noqa PLR2004
+    assert isinstance(sanitized["rrule"]["UNTIL"], datetime)
+    assert sanitized["rrule"]["BYDAY"] == ["MO", "WE", "FR"]