diff --git a/pyproject.toml b/pyproject.toml
index 919a0a8..b27a074 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -51,3 +51,18 @@ ignore = ["tests"]
 typeCheckingMode = "standard"
 useLibraryCodeForTypes = true
 reportMissingParameterType = true
+
+[tool.ruff]
+include = ["pyproject.toml", "src/**/*.py"]
+
+[tool.ruff.lint]
+select = [
+    "E4",  # pycodestyle
+    "E7",  # pycodestyle
+    "E9",  # pycodestyle
+    "F",   # Pyflakes
+    "ANN", # flake8
+]
+ignore = [
+    "ANN401", # any-type
+]
diff --git a/src/fast_forward/index/__init__.py b/src/fast_forward/index/__init__.py
index 3902de7..f1c307f 100644
--- a/src/fast_forward/index/__init__.py
+++ b/src/fast_forward/index/__init__.py
@@ -304,7 +304,7 @@ def _compute_scores(
         else:
             op = itemgetter(0)
 
-        def _mapfunc(i: int):
+        def _mapfunc(i: int) -> float:
             scores_i = select_scores[i]
             if len(scores_i) == 0:
                 return np.nan
@@ -426,7 +426,7 @@ def __call__(
         # batch encode queries
         query_vectors = self.encode_queries(list(query_df["query"]))
 
-        def _get_result(df: pd.DataFrame):
+        def _get_result(df: pd.DataFrame) -> pd.DataFrame:
             if early_stopping is None:
                 return self._compute_scores(df, query_vectors)
 
diff --git a/src/fast_forward/util/__init__.py b/src/fast_forward/util/__init__.py
index 925d0b0..f47af07 100644
--- a/src/fast_forward/util/__init__.py
+++ b/src/fast_forward/util/__init__.py
@@ -53,7 +53,7 @@ def create_coalesced_index(
     if len(target_index) > 0:
         raise ValueError("Target index is not empty.")
 
-    def _coalesce(P: np.ndarray):
+    def _coalesce(P: np.ndarray) -> list[np.ndarray]:
         P_new = []
         A = []
         A_avg = np.empty(())