diff --git a/Makefile b/Makefile
index 6c607e1..0ddaaf7 100644
--- a/Makefile
+++ b/Makefile
@@ -1,2 +1,2 @@
 verify:
-	@python3 -m compileall -q src && echo "verification passed"
+	@python3 -m compileall -q src tests && python3 -m unittest discover -s tests -q && echo "verification passed"
diff --git a/tests/__pycache__/test_calculator.cpython-315.pyc b/tests/__pycache__/test_calculator.cpython-315.pyc
new file mode 100644
index 0000000..328beb2
Binary files /dev/null and b/tests/__pycache__/test_calculator.cpython-315.pyc differ
diff --git a/tests/test_calculator.py b/tests/test_calculator.py
new file mode 100644
index 0000000..f05e23b
--- /dev/null
+++ b/tests/test_calculator.py
@@ -0,0 +1,58 @@
+# retoor <retoor@molodetz.nl>
+
+import unittest
+
+from src.calculator import add, subtract, multiply, divide
+
+
+class TestCalculator(unittest.TestCase):
+
+    def test_add(self) -> None:
+        self.assertEqual(add(2, 3), 5)
+
+    def test_subtract(self) -> None:
+        self.assertEqual(subtract(5, 3), 2)
+
+    def test_multiply_positive(self) -> None:
+        self.assertEqual(multiply(2, 3), 6)
+
+    def test_multiply_negative_left(self) -> None:
+        self.assertEqual(multiply(-2, 3), -6)
+
+    def test_multiply_negative_right(self) -> None:
+        self.assertEqual(multiply(2, -3), -6)
+
+    def test_multiply_both_negative(self) -> None:
+        self.assertEqual(multiply(-2, -3), 6)
+
+    def test_multiply_zero_left(self) -> None:
+        self.assertEqual(multiply(0, 5), 0)
+
+    def test_multiply_zero_right(self) -> None:
+        self.assertEqual(multiply(5, 0), 0)
+
+    def test_multiply_both_zero(self) -> None:
+        self.assertEqual(multiply(0, 0), 0)
+
+    def test_divide_positive(self) -> None:
+        self.assertEqual(divide(6, 3), 2)
+
+    def test_divide_negative_left(self) -> None:
+        self.assertEqual(divide(-6, 3), -2)
+
+    def test_divide_negative_right(self) -> None:
+        self.assertEqual(divide(6, -3), -2)
+
+    def test_divide_both_negative(self) -> None:
+        self.assertEqual(divide(-6, -3), 2)
+
+    def test_divide_floor(self) -> None:
+        self.assertEqual(divide(7, 3), 2)
+
+    def test_divide_zero_numerator(self) -> None:
+        self.assertEqual(divide(0, 5), 0)
+
+    def test_divide_by_zero(self) -> None:
+        with self.assertRaises(ValueError) as cm:
+            divide(5, 0)
+        self.assertEqual(str(cm.exception), 'division by zero')