diff --git a/shiftregister/app/tests.py b/shiftregister/app/tests.py
index 7ce503c..b607e8b 100644
--- a/shiftregister/app/tests.py
+++ b/shiftregister/app/tests.py
@@ -1,3 +1,142 @@
 from django.test import TestCase
+from datetime import timedelta
+from django.utils import timezone
 
-# Create your tests here.
+from .models import Helper, Room, Shift, ShiftRegistration
+
+
+class ShiftOverlapTests(TestCase):
+    def setUp(self):
+        # Create a room
+        self.room = Room.objects.create(
+            name="Test Room", required_helpers=1, meeting_location="Test Location"
+        )
+
+        # Create a helper
+        self.helper = Helper.objects.create(
+            phone="+491234567890", name="Test Helper", number_validated=True
+        )
+
+        # Create a base shift for testing overlaps
+        self.base_shift = Shift.objects.create(
+            room=self.room,
+            start_at=timezone.now() + timedelta(hours=1),
+            duration=timedelta(hours=2),
+            required_helpers=1,
+        )
+
+        # Register the helper for the base shift
+        self.base_registration = ShiftRegistration.objects.create(
+            shift=self.base_shift,
+            helper=self.helper,
+            state=ShiftRegistration.RegState.REGISTERED,
+        )
+
+    def test_no_overlap_before(self):
+        """Test a shift that ends before the base shift starts"""
+        shift_before = Shift.objects.create(
+            room=self.room,
+            start_at=self.base_shift.start_at - timedelta(hours=3),
+            duration=timedelta(hours=1),
+            required_helpers=1,
+        )
+        self.assertIsNone(self.helper.has_overlapping_shift(shift_before))
+
+    def test_no_overlap_after(self):
+        """Test a shift that starts after the base shift ends"""
+        shift_after = Shift.objects.create(
+            room=self.room,
+            start_at=self.base_shift.start_at + timedelta(hours=4),
+            duration=timedelta(hours=1),
+            required_helpers=1,
+        )
+        self.assertIsNone(self.helper.has_overlapping_shift(shift_after))
+
+    def test_overlap_start(self):
+        """Test a shift that starts during the base shift"""
+        shift_overlap_start = Shift.objects.create(
+            room=self.room,
+            start_at=self.base_shift.start_at + timedelta(hours=1),
+            duration=timedelta(hours=2),
+            required_helpers=1,
+        )
+        self.assertIsNotNone(self.helper.has_overlapping_shift(shift_overlap_start))
+
+    def test_overlap_end(self):
+        """Test a shift that ends during the base shift"""
+        shift_overlap_end = Shift.objects.create(
+            room=self.room,
+            start_at=self.base_shift.start_at - timedelta(hours=1),
+            duration=timedelta(hours=2),
+            required_helpers=1,
+        )
+        self.assertIsNotNone(self.helper.has_overlapping_shift(shift_overlap_end))
+
+    def test_overlap_contained(self):
+        """Test a shift that is completely contained within the base shift"""
+        shift_contained = Shift.objects.create(
+            room=self.room,
+            start_at=self.base_shift.start_at + timedelta(minutes=30),
+            duration=timedelta(hours=1),
+            required_helpers=1,
+        )
+        self.assertIsNotNone(self.helper.has_overlapping_shift(shift_contained))
+
+    def test_overlap_contains(self):
+        """Test a shift that completely contains the base shift"""
+        shift_contains = Shift.objects.create(
+            room=self.room,
+            start_at=self.base_shift.start_at - timedelta(hours=1),
+            duration=timedelta(hours=4),
+            required_helpers=1,
+        )
+        self.assertIsNotNone(self.helper.has_overlapping_shift(shift_contains))
+
+    def test_exact_same_time(self):
+        """Test a shift that has exactly the same time as the base shift"""
+        shift_same_time = Shift.objects.create(
+            room=self.room,
+            start_at=self.base_shift.start_at,
+            duration=self.base_shift.duration,
+            required_helpers=1,
+        )
+        self.assertIsNotNone(self.helper.has_overlapping_shift(shift_same_time))
+
+    def test_deleted_shift_no_overlap(self):
+        """Test that deleted shifts are not considered for overlap"""
+        self.base_shift.deleted = True
+        self.base_shift.save()
+
+        shift_same_time = Shift.objects.create(
+            room=self.room,
+            start_at=self.base_shift.start_at,
+            duration=self.base_shift.duration,
+            required_helpers=1,
+        )
+        self.assertIsNone(self.helper.has_overlapping_shift(shift_same_time))
+
+    def test_cancelled_registration_no_overlap(self):
+        """Test that cancelled registrations are not considered for overlap"""
+        self.base_registration.state = ShiftRegistration.RegState.CANCELED
+        self.base_registration.save()
+
+        shift_same_time = Shift.objects.create(
+            room=self.room,
+            start_at=self.base_shift.start_at,
+            duration=self.base_shift.duration,
+            required_helpers=1,
+        )
+        self.assertIsNone(self.helper.has_overlapping_shift(shift_same_time))
+
+    def test_failed_registration_no_overlap(self):
+        """Test that failed registrations are not considered for overlap"""
+        self.base_registration.state = ShiftRegistration.RegState.FAILED
+        self.base_registration.save()
+
+        shift_same_time = Shift.objects.create(
+            room=self.room,
+            start_at=self.base_shift.start_at,
+            duration=self.base_shift.duration,
+            required_helpers=1,
+        )
+        self.assertIsNone(self.helper.has_overlapping_shift(shift_same_time))