diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
index a531570..bb06934 100644
--- a/.pre-commit-config.yaml
+++ b/.pre-commit-config.yaml
@@ -42,4 +42,18 @@ repos:
       - id: bandit
         entry: bandit
         exclude: ^tests/
+  - repo: https://github.com/pre-commit/mirrors-mypy
+    rev: v1.10.0
+    hooks:
+      - id: mypy
+        # TODO: Get rid of Any types. mypy doesn't take jsonschema.validate
+        # into account
+        # args:
+        #   - --strict
+        additional_dependencies:
+          - pandas-stubs
+          - types-jsonschema
+          - types-PyYAML
+        files: ^aws_project_costs
+
 exclude: "^tests/dummy-.*/.*.md$"
diff --git a/aws_project_costs/main.py b/aws_project_costs/main.py
index f95d798..9688fa3 100644
--- a/aws_project_costs/main.py
+++ b/aws_project_costs/main.py
@@ -6,7 +6,7 @@
 from .schema import validate
 
 
-def main():
+def main() -> None:
     parser = ArgumentParser("aws-project-costs")
     parser.add_argument(
         "--config", required=True, help="Project costs configuration file"
diff --git a/aws_project_costs/project_costs.py b/aws_project_costs/project_costs.py
index bb46a47..6388fae 100644
--- a/aws_project_costs/project_costs.py
+++ b/aws_project_costs/project_costs.py
@@ -1,6 +1,7 @@
 import json
 from enum import Enum
 from operator import itemgetter
+from typing import Any, Hashable, Optional
 
 import pandas as pd
 
@@ -12,19 +13,21 @@ class CostSourceType(Enum):
     SHARED = "shared"
 
 
-def _get_account_cfg(config, accountname):
+def _get_account_cfg(config: dict[str, Any], accountname: str) -> dict[str, Any]:
     for acc in config["accounts"]:
         if acc["name"].lower() == accountname.lower():
             return acc
     raise ValueError(f"Configuration for account {accountname} not found")
 
 
-def _get_project_costshares_in_groups(config, groupnames):
+def _get_project_costshares_in_groups(
+    config: dict[str, Any], groupnames: list[str]
+) -> dict[str, int]:
     """
     Get the combined set of all projects in groupnames, and their costshare
     If a project is in multiple groups it must have the same costshare
     """
-    projects = dict()
+    projects: dict[str, int] = dict()
     for group in groupnames:
         for p in config["project-groups"][group]:
             name = p["name"]
@@ -38,7 +41,12 @@ def _get_project_costshares_in_groups(config, groupnames):
     return projects
 
 
-def _project_specific_account(start, description, project_name, costs_dict):
+def _project_specific_account(
+    start: pd.Timestamp,
+    description: str,
+    project_name: str,
+    costs_dict: list[dict[Hashable, Any]],
+) -> list[tuple[str, str, str, Any, str, Any]]:
     rows = []
 
     for item in costs_dict:
@@ -57,14 +65,14 @@ def _project_specific_account(start, description, project_name, costs_dict):
 
 
 def _shared_account(
-    start,
-    description,
-    proj_tag_names_map,
-    shared_tag_values,
-    project_tagname,
-    projects_in_group,
-    costs_dict,
-):
+    start: pd.Timestamp,
+    description: str,
+    proj_tag_names_map: dict[str, str],
+    shared_tag_values: list[str],
+    project_tagname: Optional[str],
+    projects_in_group: dict[str, int],
+    costs_dict: list[dict[Hashable, Any]],
+) -> list[tuple[str, str, str, Any, str, Any]]:
     rows = []
 
     for item in costs_dict:
@@ -105,7 +113,9 @@ def _shared_account(
     return rows
 
 
-def allocate_costs(*, accountname, config, start, df):
+def allocate_costs(
+    *, accountname: str, config: dict[str, Any], start: pd.Timestamp, df: pd.DataFrame
+) -> list[tuple[str, str, str, Any, str, Any]]:
     account_cfg = _get_account_cfg(config, accountname)
 
     costs = df[(df["START"] == start) & (df["accountname"] == accountname)]
@@ -150,7 +160,11 @@ def allocate_costs(*, accountname, config, start, df):
     return rows
 
 
-def analyse_costs_csv(config, costs_csv_filename, output_csv_filename=None):
+def analyse_costs_csv(
+    config: dict[str, Any],
+    costs_csv_filename: str,
+    output_csv_filename: Optional[str] = None,
+) -> None:
     df = pd.read_csv(
         costs_csv_filename,
         dtype={"accountname": str, f"{PROJECT_TAG}$": str, "COST": float},
diff --git a/aws_project_costs/schema.py b/aws_project_costs/schema.py
index 59616e8..3b88cc9 100644
--- a/aws_project_costs/schema.py
+++ b/aws_project_costs/schema.py
@@ -1,12 +1,13 @@
 import json
 import sys
 from pathlib import Path
+from typing import Any
 
 import jsonschema
 import yaml
 
 
-def validate(projects, raise_on_error=False):
+def validate(projects: dict[str, Any], raise_on_error: bool = False) -> list[str]:
     with (Path(__file__).resolve().parent / "project-cost-schema.json").open() as f:
         schema = json.load(f)