diff --git a/tests/test_clean.py b/tests/test_clean.py
new file mode 100644
index 0000000..03c7598
--- /dev/null
+++ b/tests/test_clean.py
@@ -0,0 +1,23 @@
+import pandas as pd
+import pytest
+from src.transform.clean import drop_nulls, deduplicate, cast_types
+
+
+def test_drop_nulls_removes_rows():
+    df = pd.DataFrame({"id": [1, 2, None], "val": ["a", "b", "c"]})
+    result = drop_nulls(df, ["id"])
+    assert len(result) == 2
+    assert 3 not in result["id"].values
+
+
+def test_deduplicate_keeps_last():
+    df = pd.DataFrame({"id": [1, 1, 2], "val": ["old", "new", "only"]})
+    result = deduplicate(df, ["id"])
+    assert len(result) == 2
+    assert result[result["id"] == 1]["val"].values[0] == "new"
+
+
+def test_cast_types():
+    df = pd.DataFrame({"count": ["1", "2", "3"]})
+    result = cast_types(df, {"count": int})
+    assert result["count"].dtype == int