diff --git a/rosidl_generator_c/rosidl_generator_c/__init__.py b/rosidl_generator_c/rosidl_generator_c/__init__.py
index 7dcd61a40..673e69565 100644
--- a/rosidl_generator_c/rosidl_generator_c/__init__.py
+++ b/rosidl_generator_c/rosidl_generator_c/__init__.py
@@ -13,6 +13,8 @@
 # limitations under the License.
 from math import isnan
 
+from typing import List
+
 from rosidl_generator_type_description import parse_rihs_string
 from rosidl_generator_type_description import RIHS01_HASH_VALUE_SIZE
 from rosidl_parser.definition import AbstractGenericString
@@ -29,7 +31,7 @@
 from rosidl_pycommon import generate_files
 
 
-def generate_c(generator_arguments_file, disable_description_codegen=False):
+def generate_c(generator_arguments_file, disable_description_codegen: bool = False) -> List[str]:
     mapping = {
         'idl.h.em': '%s.h',
         'idl__description.c.em': 'detail/%s__description.c',
@@ -47,7 +49,7 @@ def generate_c(generator_arguments_file, disable_description_codegen=False):
         })
 
 
-def prefix_with_bom_if_necessary(content):
+def prefix_with_bom_if_necessary(content: str) -> str:
     try:
         content.encode('ASCII')
     except UnicodeError:
diff --git a/rosidl_generator_cpp/rosidl_generator_cpp/__init__.py b/rosidl_generator_cpp/rosidl_generator_cpp/__init__.py
index f138bcf10..ed6f70962 100644
--- a/rosidl_generator_cpp/rosidl_generator_cpp/__init__.py
+++ b/rosidl_generator_cpp/rosidl_generator_cpp/__init__.py
@@ -14,6 +14,7 @@
 
 from ast import literal_eval
 from math import isnan
+from typing import List
 
 from rosidl_parser.definition import AbstractGenericString
 from rosidl_parser.definition import AbstractNestedType
@@ -29,7 +30,7 @@
 from rosidl_pycommon import generate_files
 
 
-def generate_cpp(generator_arguments_file):
+def generate_cpp(generator_arguments_file) -> List[str]:
     mapping = {
         'idl.hpp.em': '%s.hpp',
         'idl__builder.hpp.em': 'detail/%s__builder.hpp',
@@ -42,7 +43,7 @@ def generate_cpp(generator_arguments_file):
         post_process_callback=prefix_with_bom_if_necessary)
 
 
-def prefix_with_bom_if_necessary(content):
+def prefix_with_bom_if_necessary(content: str) -> str:
     try:
         content.encode('ASCII')
     except UnicodeError:
diff --git a/rosidl_pycommon/package.xml b/rosidl_pycommon/package.xml
index 9467b4a68..30a49c26b 100644
--- a/rosidl_pycommon/package.xml
+++ b/rosidl_pycommon/package.xml
@@ -20,6 +20,7 @@
   <test_depend>ament_copyright</test_depend>
   <test_depend>ament_flake8</test_depend>
   <test_depend>ament_pep257</test_depend>
+  <test_depend>ament_mypy</test_depend>
   <test_depend>python3-pytest</test_depend>
 
   <export>
diff --git a/rosidl_pycommon/py.typed b/rosidl_pycommon/py.typed
new file mode 100644
index 000000000..e69de29bb
diff --git a/rosidl_pycommon/rosidl_pycommon/__init__.py b/rosidl_pycommon/rosidl_pycommon/__init__.py
index 36d887113..d52ac9608 100644
--- a/rosidl_pycommon/rosidl_pycommon/__init__.py
+++ b/rosidl_pycommon/rosidl_pycommon/__init__.py
@@ -18,6 +18,7 @@
 import pathlib
 import re
 import sys
+from typing import Any, Callable, Dict, List, Optional
 
 import em
 
@@ -31,7 +32,7 @@
 from rosidl_parser.parser import parse_idl_file
 
 
-def convert_camel_case_to_lower_case_underscore(value):
+def convert_camel_case_to_lower_case_underscore(value: str) -> str:
     # insert an underscore before any upper case letter
     # which is followed by a lower case letter
     value = re.sub('(.)([A-Z][a-z]+)', r'\1_\2', value)
@@ -41,12 +42,14 @@ def convert_camel_case_to_lower_case_underscore(value):
     return value.lower()
 
 
-def read_generator_arguments(input_file):
+def read_generator_arguments(input_file: str) -> Any:
     with open(input_file, mode='r', encoding='utf-8') as h:
         return json.load(h)
 
 
-def get_newest_modification_time(target_dependencies):
+def get_newest_modification_time(
+    target_dependencies: List[str]
+) -> Optional[float]:
     newest_timestamp = None
     for dep in target_dependencies:
         ts = os.path.getmtime(dep)
@@ -56,9 +59,10 @@ def get_newest_modification_time(target_dependencies):
 
 
 def generate_files(
-    generator_arguments_file, mapping, additional_context=None,
-    keep_case=False, post_process_callback=None
-):
+    generator_arguments_file: str, mapping: Dict[str, str],
+    additional_context: Optional[Dict[str, bool]] = None,
+    keep_case: bool = False, post_process_callback: Optional[Callable[[str], str]] = None
+) -> List[str]:
     args = read_generator_arguments(generator_arguments_file)
 
     template_basepath = pathlib.Path(args['template_dir'])
@@ -67,7 +71,7 @@ def generate_files(
             'Could not find template: ' + template_filename
 
     latest_target_timestamp = get_newest_modification_time(args['target_dependencies'])
-    generated_files = []
+    generated_files: List[str] = []
 
     type_description_files = {}
     for description_tuple in args.get('type_description_tuples', []):
@@ -128,10 +132,10 @@ def generate_files(
     return generated_files
 
 
-template_prefix_path = []
+template_prefix_path: List[pathlib.Path] = []
 
 
-def get_template_path(template_name):
+def get_template_path(template_name: str) -> pathlib.Path:
     global template_prefix_path
     for basepath in template_prefix_path:
         template_path = basepath / template_name
@@ -144,14 +148,16 @@ def get_template_path(template_name):
 
 
 def expand_template(
-    template_name, data, output_file, minimum_timestamp=None,
-    template_basepath=None, post_process_callback=None
-):
+    template_name: str, data: Dict[str, Any], output_file: str,
+    minimum_timestamp: Optional[float] = None,
+    template_basepath: Optional[pathlib.Path] = None,
+    post_process_callback: Optional[Callable[[str], str]] = None
+) -> None:
     # in the legacy API the first argument was the path to the template
     if template_basepath is None:
-        template_name = pathlib.Path(template_name)
-        template_basepath = template_name.parent
-        template_name = template_name.name
+        template_path = pathlib.Path(template_name)
+        template_basepath = template_path.parent
+        template_name = template_path.name
 
     global template_prefix_path
     template_prefix_path.append(template_basepath)
@@ -226,14 +232,17 @@ def expand_template(
         h.write(content)
 
 
-def _add_helper_functions(data):
+def _add_helper_functions(data: Dict[str, Any]) -> None:
     data['TEMPLATE'] = _expand_template
 
 
-def _expand_template(template_name, **kwargs):
+def _expand_template(template_name: str, **kwargs: Any) -> None:
     global interpreter
     template_path = get_template_path(template_name)
     _add_helper_functions(kwargs)
+    if interpreter is None:
+        raise RuntimeError('_expand_template called before expand_template')
+
     with template_path.open('r') as h:
         interpreter.invoke(
             'beforeInclude', name=str(template_path), file=h, locals=kwargs)
diff --git a/rosidl_pycommon/test/test_copyright.py b/rosidl_pycommon/test/test_copyright.py
index a89fa2726..2db7dabb1 100644
--- a/rosidl_pycommon/test/test_copyright.py
+++ b/rosidl_pycommon/test/test_copyright.py
@@ -18,6 +18,6 @@
 
 @pytest.mark.copyright
 @pytest.mark.linter
-def test_copyright():
+def test_copyright() -> None:
     rc = main(argv=['.', 'test'])
     assert rc == 0, 'Found errors'
diff --git a/rosidl_pycommon/test/test_flake8.py b/rosidl_pycommon/test/test_flake8.py
index 648bb70f9..7830616b3 100644
--- a/rosidl_pycommon/test/test_flake8.py
+++ b/rosidl_pycommon/test/test_flake8.py
@@ -18,7 +18,7 @@
 
 @pytest.mark.flake8
 @pytest.mark.linter
-def test_flake8():
+def test_flake8() -> None:
     rc, errors = main_with_errors(argv=[])
     assert rc == 0, \
         'Found %d code style errors / warnings:\n' % len(errors) + \
diff --git a/rosidl_pycommon/test/test_mypy.py b/rosidl_pycommon/test/test_mypy.py
new file mode 100644
index 000000000..97e4f502a
--- /dev/null
+++ b/rosidl_pycommon/test/test_mypy.py
@@ -0,0 +1,23 @@
+# Copyright 2024 Open Source Robotics Foundation, Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from ament_mypy.main import main
+import pytest
+
+
+@pytest.mark.mypy
+@pytest.mark.linter
+def test_mypy() -> None:
+    rc = main(argv=[])
+    assert rc == 0, 'Found type errors!'
diff --git a/rosidl_pycommon/test/test_pep257.py b/rosidl_pycommon/test/test_pep257.py
index 7a2a3747e..d1d2b8e95 100644
--- a/rosidl_pycommon/test/test_pep257.py
+++ b/rosidl_pycommon/test/test_pep257.py
@@ -18,6 +18,6 @@
 
 @pytest.mark.linter
 @pytest.mark.pep257
-def test_pep257():
+def test_pep257() -> None:
     rc = main(argv=['.', 'test'])
     assert rc == 0, 'Found code style errors / warnings'
diff --git a/rosidl_typesupport_introspection_c/rosidl_typesupport_introspection_c/__init__.py b/rosidl_typesupport_introspection_c/rosidl_typesupport_introspection_c/__init__.py
index 8a728f9a5..ab96f68d2 100644
--- a/rosidl_typesupport_introspection_c/rosidl_typesupport_introspection_c/__init__.py
+++ b/rosidl_typesupport_introspection_c/rosidl_typesupport_introspection_c/__init__.py
@@ -12,10 +12,12 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from typing import List
+
 from rosidl_pycommon import generate_files
 
 
-def generate_c(generator_arguments_file: str):
+def generate_c(generator_arguments_file: str) -> List[str]:
     """
     Generate the C implementation of the type support.
 
diff --git a/rosidl_typesupport_introspection_cpp/rosidl_typesupport_introspection_cpp/__init__.py b/rosidl_typesupport_introspection_cpp/rosidl_typesupport_introspection_cpp/__init__.py
index 7457e2a60..b4ba5ed45 100644
--- a/rosidl_typesupport_introspection_cpp/rosidl_typesupport_introspection_cpp/__init__.py
+++ b/rosidl_typesupport_introspection_cpp/rosidl_typesupport_introspection_cpp/__init__.py
@@ -12,10 +12,12 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from typing import List
+
 from rosidl_pycommon import generate_files
 
 
-def generate_cpp(generator_arguments_file: str):
+def generate_cpp(generator_arguments_file: str) -> List[str]:
     """
     Generate the C++ implementation of the type support.
 
diff --git a/rosidl_typesupport_introspection_cpp/rosidl_typesupport_introspection_cpp/cli.py b/rosidl_typesupport_introspection_cpp/rosidl_typesupport_introspection_cpp/cli.py
index a26b5d99d..d9091680d 100644
--- a/rosidl_typesupport_introspection_cpp/rosidl_typesupport_introspection_cpp/cli.py
+++ b/rosidl_typesupport_introspection_cpp/rosidl_typesupport_introspection_cpp/cli.py
@@ -13,6 +13,7 @@
 # limitations under the License.
 
 import pathlib
+from typing import List
 
 from ament_index_python import get_package_share_directory
 
@@ -32,7 +33,7 @@ def generate(
         interface_files,
         include_paths,
         output_path
-    ):
+    ) -> List[str]:
         package_share_path = pathlib.Path(
             get_package_share_directory('rosidl_typesupport_introspection_cpp'))