diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
index 4fad14e..2aa1cd4 100644
--- a/.pre-commit-config.yaml
+++ b/.pre-commit-config.yaml
@@ -22,3 +22,8 @@ repos:
   hooks:
     - id: djlint-jinja
       types_or: ['html', 'jinja']
+- repo: https://github.com/pre-commit/mirrors-mypy
+  rev: 'v1.10.0'  # Use the sha / tag you want to point at
+  hooks:
+  -   id: mypy
+      additional_dependencies: [types-jmespath, types-requests, types-beautifulsoup4, types-flask]
diff --git a/app/__init__.py b/app/__init__.py
index dad7d1f..33055ae 100644
--- a/app/__init__.py
+++ b/app/__init__.py
@@ -41,7 +41,7 @@ def create_app(config_class=Config) -> Flask:
     logging.init_app(app)
     app.jinja_env.lstrip_blocks = True
     app.jinja_env.trim_blocks = True
-    app.jinja_loader = ChoiceLoader(
+    app.jinja_loader = ChoiceLoader(  # type: ignore # TODO: fixme
         [
             PackageLoader("app"),
             PrefixLoader(
@@ -111,8 +111,8 @@ def setup_funds_and_auth(app: Flask) -> None:
 
     app.config["AUTH_MAPPINGS"] = AuthService(
         fund_to_auth_mappings={
-            towns_fund.fund_name: AuthMapping(towns_fund.auth_class, tf_auth),
-            pathfinders.fund_name: AuthMapping(pathfinders.auth_class, pf_auth),
+            towns_fund.fund_name: AuthMapping(towns_fund.auth_class, tf_auth),  # type: ignore # TODO: fixme
+            pathfinders.fund_name: AuthMapping(pathfinders.auth_class, pf_auth),  # type: ignore # TODO: fixme
         }
     )
 
diff --git a/app/main/data_requests.py b/app/main/data_requests.py
index f726173..d41922e 100644
--- a/app/main/data_requests.py
+++ b/app/main/data_requests.py
@@ -6,7 +6,7 @@
 from config import Config
 
 
-def post_ingest(file: FileStorage, data: dict = None) -> tuple[dict | None, dict | None, dict | None]:
+def post_ingest(file: FileStorage, data: dict | None = None) -> tuple[dict | None, dict | None, dict | None]:
     """Send an HTTP POST request to ingest into the data store
      server and return the response.
 
diff --git a/app/main/notify.py b/app/main/notify.py
index ea07d1d..da43054 100644
--- a/app/main/notify.py
+++ b/app/main/notify.py
@@ -1,5 +1,4 @@
 from datetime import datetime
-from typing import BinaryIO
 
 import notifications_python_client as notify
 from flask import current_app
@@ -16,7 +15,7 @@
 def send_email(
     email_address: str,
     template_id: str,
-    file: BinaryIO | FileStorage = None,
+    file: FileStorage | None = None,
     **kwargs,
 ) -> None:
     """Send email to the specified email address via the GovUK Notify service.
diff --git a/app/utils.py b/app/utils.py
index db8c14e..caf2412 100644
--- a/app/utils.py
+++ b/app/utils.py
@@ -6,7 +6,7 @@
 from config import Config
 
 
-def get_friendly_fund_type(fund_type_id: str) -> str | None:
+def get_friendly_fund_type(fund_type_id) -> str | None:
     """Gets a friendly fund type name from an ID.
 
     If fund_type_id is not recognised, it is logged and None is returned.
@@ -19,6 +19,8 @@ def get_friendly_fund_type(fund_type_id: str) -> str | None:
     except KeyError:
         current_app.logger.error("Unknown fund type id found: {fund_type_id}", extra=dict(fund_type_id=fund_type_id))
 
+    return None
+
 
 def days_between_dates(date1: date, date2: date) -> int:
     """Calculate the number of days between two dates.
diff --git a/config/__init__.py b/config/__init__.py
index c2762fe..42d09ec 100644
--- a/config/__init__.py
+++ b/config/__init__.py
@@ -10,15 +10,15 @@
     case "unit_test":
         from config.envs.unit_test import UnitTestConfig as Config
     case "development":
-        from config.envs.development import DevelopmentConfig as Config
+        from config.envs.development import DevelopmentConfig as Config  # type: ignore # TODO: fixme
     case "dev" | "test" | "production":
-        from config.envs.aws import AwsConfig as Config
+        from config.envs.aws import AwsConfig as Config  # type: ignore # TODO: fixme
     case _:
-        from config.envs.default import DefaultConfig as Config
+        from config.envs.default import DefaultConfig as Config  # type: ignore # TODO: fixme
 
 try:
-    Config.pretty_print()
+    Config.pretty_print()  # type: ignore # TODO: fixme
 except AttributeError:
     print({"msg": "Config doesn't have pretty_print function."})
 
-__all__ = [Config]
+__all__ = ["Config"]
diff --git a/config/envs/default.py b/config/envs/default.py
index 4605489..24a29e2 100644
--- a/config/envs/default.py
+++ b/config/envs/default.py
@@ -40,7 +40,7 @@ class DefaultConfig(object):
     # RSA 256 KEYS
     RSA256_PUBLIC_KEY_BASE64 = os.getenv("RSA256_PUBLIC_KEY_BASE64")
     if RSA256_PUBLIC_KEY_BASE64:
-        RSA256_PUBLIC_KEY = base64.b64decode(RSA256_PUBLIC_KEY_BASE64).decode()
+        RSA256_PUBLIC_KEY: str = base64.b64decode(RSA256_PUBLIC_KEY_BASE64).decode()
 
     TF_ADDITIONAL_EMAIL_LOOKUPS = ast.literal_eval(os.getenv("TF_ADDITIONAL_EMAIL_LOOKUPS", "{}"))
     if not isinstance(TF_ADDITIONAL_EMAIL_LOOKUPS, dict):
diff --git a/config/envs/development.py b/config/envs/development.py
index a4417c1..7cccdde 100644
--- a/config/envs/development.py
+++ b/config/envs/development.py
@@ -10,7 +10,7 @@ class DevelopmentConfig(DefaultConfig):
     # RSA 256 KEYS
     if not hasattr(DefaultConfig, "RSA256_PUBLIC_KEY"):
         _test_public_key_path = DefaultConfig.FLASK_ROOT + "/tests/keys/rsa256/public.pem"
-        with open(_test_public_key_path, mode="rb") as public_key_file:
+        with open(_test_public_key_path, mode="r") as public_key_file:
             RSA256_PUBLIC_KEY = public_key_file.read()
 
     # devs can submit for these LAs and places
diff --git a/config/envs/unit_test.py b/config/envs/unit_test.py
index aee0273..ea258c5 100644
--- a/config/envs/unit_test.py
+++ b/config/envs/unit_test.py
@@ -9,7 +9,7 @@ class UnitTestConfig(DefaultConfig):
     # RSA 256 KEYS
     if not hasattr(DefaultConfig, "RSA256_PUBLIC_KEY"):
         _test_public_key_path = DefaultConfig.FLASK_ROOT + "/tests/keys/rsa256/public.pem"
-        with open(_test_public_key_path, mode="rb") as public_key_file:
+        with open(_test_public_key_path, mode="r") as public_key_file:
             RSA256_PUBLIC_KEY = public_key_file.read()
 
     EXAMPLE_INGEST_WRONG_FORMAT = DefaultConfig.FLASK_ROOT + "/tests/resources/wrong_format_test_file.txt"
diff --git a/app.py b/main_app.py
similarity index 100%
rename from app.py
rename to main_app.py
diff --git a/pyproject.toml b/pyproject.toml
index ae0ee1e..abdda0a 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -21,3 +21,14 @@ exclude = [
     "cache",
 ]
 mccabe.max-complexity = 12
+
+[tool.mypy]
+python_version = "3.11"
+
+[[tool.mypy.overrides]]
+module = [
+  "fsd_utils.*",
+  "notifications_python_client.*",
+  "flask_assets"
+]
+ignore_missing_imports = true
diff --git a/requirements-dev.in b/requirements-dev.in
index bf9f135..3a61e4d 100644
--- a/requirements-dev.in
+++ b/requirements-dev.in
@@ -10,6 +10,7 @@ pip-tools==6.14.0
 ruff
 pep8-naming==0.13.3
 pur==7.1.0
+mypy
 
 #-----------------------------------
 #   Testing
diff --git a/requirements-dev.txt b/requirements-dev.txt
index 8a294bd..e35332a 100644
--- a/requirements-dev.txt
+++ b/requirements-dev.txt
@@ -157,6 +157,10 @@ markupsafe==2.1.3
     #   werkzeug
 mccabe==0.7.0
     # via flake8
+mypy==1.10.0
+    # via -r requirements-dev.in
+mypy-extensions==1.0.0
+    # via mypy
 notifications-python-client==8.1.0
     # via -r requirements.txt
 packaging==23.1
@@ -288,6 +292,7 @@ typing-extensions==4.7.1
     # via
     #   -r requirements.txt
     #   alembic
+    #   mypy
     #   sqlalchemy
 urllib3==1.26.18
     # via
diff --git a/static_assets.py b/static_assets.py
index 50a404e..679b98b 100644
--- a/static_assets.py
+++ b/static_assets.py
@@ -32,7 +32,3 @@ def build_bundles(static_folder):
     bundles = init_assets(static_folder=static_folder)
     for bundle in bundles:
         bundle.build()
-
-
-if __name__ == "__main__":
-    build_bundles()
diff --git a/tests/conftest.py b/tests/conftest.py
index 00c3896..6ca3bbd 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -1,3 +1,5 @@
+from typing import Generator
+
 import pytest
 from flask.testing import FlaskClient
 from werkzeug.datastructures import FileStorage
@@ -48,7 +50,7 @@ def mocked_pf_and_tf_auth(mocker):
 
 
 @pytest.fixture()
-def flask_test_client(mocked_auth) -> FlaskClient:
+def flask_test_client(mocked_auth) -> Generator[FlaskClient, None, None]:
     """
     Creates the test client we will be using to test the responses
     from our app, this is a test fixture.
@@ -62,7 +64,7 @@ def flask_test_client(mocked_auth) -> FlaskClient:
 
 
 @pytest.fixture()
-def unauthenticated_flask_test_client() -> FlaskClient:
+def unauthenticated_flask_test_client() -> Generator[FlaskClient, None, None]:
     """
     :return: An unauthenticated flask test client.
     """
@@ -71,14 +73,14 @@ def unauthenticated_flask_test_client() -> FlaskClient:
 
 
 @pytest.fixture(scope="function")
-def example_pre_ingest_data_file() -> FileStorage:
+def example_pre_ingest_data_file() -> Generator[FileStorage, None, None]:
     """An example pre ingest spreadsheet in towns-fund format."""
     with open(UnitTestConfig.EXAMPLE_INGEST_DATA_PATH, "rb") as file:
-        yield file
+        yield FileStorage(file)
 
 
 @pytest.fixture(scope="function")
-def example_ingest_wrong_format() -> FileStorage:
+def example_ingest_wrong_format() -> Generator[FileStorage, None, None]:
     """An example pre ingest spreadsheet in towns-fund format."""
     with open(UnitTestConfig.EXAMPLE_INGEST_WRONG_FORMAT, "rb") as file:
-        yield file
+        yield FileStorage(file)