diff --git a/.github/workflows/plone-package.yml b/.github/workflows/plone-package.yml
index d8a0496..59f7823 100644
--- a/.github/workflows/plone-package.yml
+++ b/.github/workflows/plone-package.yml
@@ -33,6 +33,20 @@ jobs:
       - name: Ruff
         run: tox -e ruff-check
 
+  mypy:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+        with:
+          persist-credentials: false
+      - uses: actions/setup-python@5fda3b95a4ea91299a34e894583c3862153e4b97 # v7.0.0
+        with:
+          python-version: '3.14'
+      - name: Install tox
+        run: python -m pip install tox
+      - name: Mypy
+        run: tox -e mypy
+
   test:
     runs-on: ubuntu-latest
     needs: lint
diff --git a/AGENTS.md b/AGENTS.md
index 2bf8ced..932a0d2 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -30,6 +30,7 @@ Run the tests and the linters with tox:
 tox -e py314-Plone62      # one Python and Plone version
 tox -e ruff-check         # lint and format check
 tox -e ruff-fix           # fix and format
+tox -e mypy               # type check src and tests
 tox                       # the whole matrix
 ```
 
diff --git a/pyproject.toml b/pyproject.toml
index 7114952..891d993 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -59,6 +59,11 @@ release = [
     "zest.pocompile",
 ]
 
+[dependency-groups]
+dev = [
+    "mypy==2.4.0",
+]
+
 [project.urls]
 Homepage = "https://github.com/cmscom/c2.transform.msoffice"
 PyPI = "https://pypi.org/project/c2.transform.msoffice"
@@ -122,6 +127,8 @@ fix = true
 lint.select = [
     # flake8-2020
     "YTT",
+    # flake8-annotations
+    "ANN",
     # flake8-bandit
     "S",
     # flake8-bugbear
@@ -165,7 +172,52 @@ order-by-type = false
 preview = true
 
 [tool.ruff.lint.per-file-ignores]
-"tests/*" = ["E501", "RUF001", "S101"]
+# Test functions take pytest fixtures and return nothing; fixtures and
+# helpers in tests are annotated by convention.
+"tests/*" = ["ANN001", "ANN201", "E501", "RUF001", "S101"]
+
+[tool.mypy]
+python_version = "3.10"
+# Check the sources, not the installed package (src/c2 and src/c2/transform
+# are namespace packages).
+mypy_path = "src"
+explicit_package_bases = true
+disallow_untyped_defs = true
+check_untyped_defs = true
+warn_unused_ignores = true
+warn_redundant_casts = true
+warn_return_any = true
+warn_unused_configs = true
+
+[[tool.mypy.overrides]]
+module = ["tests.*"]
+disallow_untyped_defs = false
+
+# Zope, Plone and their test tools have no type information.
+[[tool.mypy.overrides]]
+module = [
+    "Acquisition",
+    "OFS.Application",
+    "plone",
+    "plone.app.contenttypes.content",
+    "plone.app.contenttypes.indexers",
+    "plone.app.contenttypes.testing",
+    "plone.app.testing",
+    "plone.base.interfaces",
+    "plone.namedfile.file",
+    "Products.CMFCore.utils",
+    "Products.CMFPlone.controlpanel.browser.quickinstaller",
+    "Products.CMFPlone.Portal",
+    "Products.GenericSetup.tool",
+    "Products.PortalTransforms.data",
+    "Products.PortalTransforms.interfaces",
+    "Products.PortalTransforms.TransformEngine",
+    "pytest_plone",
+    "zope.component",
+    "zope.configuration.config",
+    "zope.interface",
+]
+ignore_missing_imports = true
 
 [tool.pytest.ini_options]
 testpaths = ["tests"]
diff --git a/src/c2/transform/msoffice/setuphandlers.py b/src/c2/transform/msoffice/setuphandlers.py
index 8bef927..e39f0bc 100644
--- a/src/c2/transform/msoffice/setuphandlers.py
+++ b/src/c2/transform/msoffice/setuphandlers.py
@@ -2,6 +2,7 @@
 from c2.transform.msoffice import logger
 from plone.base.interfaces import INonInstallable
 from Products.CMFCore.utils import getToolByName
+from Products.GenericSetup.tool import SetupTool
 from zope.interface import implementer
 
 
@@ -10,12 +11,12 @@
 
 @implementer(INonInstallable)
 class HiddenProfiles:
-    def getNonInstallableProfiles(self):
+    def getNonInstallableProfiles(self) -> list[str]:
         """Hide the uninstall profile from site creation and the add-ons list."""
         return ["c2.transform.msoffice:uninstall"]
 
 
-def post_install(context):
+def post_install(context: SetupTool) -> None:
     """Register the Office MIME types and the transform.
 
     ``context`` is the setup tool. Running it again changes nothing.
@@ -47,7 +48,7 @@ def post_install(context):
     transforms.manage_addTransform(config.TRANSFORM_NAME, TRANSFORM_MODULE)
 
 
-def uninstall(context):
+def uninstall(context: SetupTool) -> None:
     """Remove the transform. The MIME types stay; others may use them."""
     transforms = getToolByName(context, "portal_transforms")
     if config.TRANSFORM_NAME not in transforms.objectIds():
diff --git a/src/c2/transform/msoffice/testing.py b/src/c2/transform/msoffice/testing.py
index 4f2d037..476fd30 100644
--- a/src/c2/transform/msoffice/testing.py
+++ b/src/c2/transform/msoffice/testing.py
@@ -1,8 +1,11 @@
+from OFS.Application import Application
 from plone.app.contenttypes.testing import PLONE_APP_CONTENTTYPES_FIXTURE
 from plone.app.testing import applyProfile
 from plone.app.testing import FunctionalTesting
 from plone.app.testing import IntegrationTesting
 from plone.app.testing import PloneSandboxLayer
+from Products.CMFPlone.Portal import PloneSite
+from zope.configuration.config import ConfigurationMachine
 
 import c2.transform.msoffice
 
@@ -10,11 +13,13 @@
 class Layer(PloneSandboxLayer):
     defaultBases = (PLONE_APP_CONTENTTYPES_FIXTURE,)
 
-    def setUpZope(self, app, configurationContext):
+    def setUpZope(
+        self, app: Application, configurationContext: ConfigurationMachine
+    ) -> None:
         # z3c.autoinclude is disabled in the Plone fixture base layer.
         self.loadZCML(package=c2.transform.msoffice)
 
-    def setUpPloneSite(self, portal):
+    def setUpPloneSite(self, portal: PloneSite) -> None:
         applyProfile(portal, "c2.transform.msoffice:default")
 
 
diff --git a/src/c2/transform/msoffice/transform.py b/src/c2/transform/msoffice/transform.py
index 0c5ecd0..4eaf8ea 100644
--- a/src/c2/transform/msoffice/transform.py
+++ b/src/c2/transform/msoffice/transform.py
@@ -4,6 +4,7 @@
 from c2.transform.msoffice.config import SITE_CHARSET
 from c2.transform.msoffice.config import TRANSFORM_NAME
 from pathlib import Path
+from Products.PortalTransforms.data import datastream
 from Products.PortalTransforms.interfaces import ITransform
 from zope.interface import implementer
 
@@ -116,16 +117,19 @@ class msoffice_to_text:
 
     output_encoding = SITE_CHARSET
 
-    def __init__(self, name=None):
+    def __init__(self, name: str | None = None) -> None:
         if name:
             self.__name__ = name
 
-    def name(self):
+    def name(self) -> str:
         return self.__name__
 
-    def convert(self, orig, data, **kwargs):
+    def convert(
+        self, orig: bytes | str, data: datastream, **kwargs: object
+    ) -> datastream:
         """Set the text of ``orig`` (bytes) on ``data``.
 
+        ``data`` is the datastream that portal_transforms made for the result.
         On any failure the text is empty, so saving the file still works.
         """
         if not isinstance(orig, bytes):
@@ -140,5 +144,5 @@ def convert(self, orig, data, **kwargs):
         return data
 
 
-def register():
+def register() -> msoffice_to_text:
     return msoffice_to_text()
diff --git a/tests/test_config.py b/tests/test_config.py
index 8233741..8cf0de4 100644
--- a/tests/test_config.py
+++ b/tests/test_config.py
@@ -6,7 +6,7 @@
 
 
 @pytest.fixture
-def stdlib_mimetypes_only(monkeypatch):
+def stdlib_mimetypes_only(monkeypatch: pytest.MonkeyPatch) -> None:
     """Forget the host's mime.types (slim images do not have one).
 
     mimetypes.init(files=[]) keeps the current database, so it is replaced.
diff --git a/tests/test_java.py b/tests/test_java.py
index 6bd1a76..6bb6a38 100644
--- a/tests/test_java.py
+++ b/tests/test_java.py
@@ -6,8 +6,10 @@
 
 from pathlib import Path
 from plone import api
+from plone.app.contenttypes.content import File
 from plone.app.contenttypes.indexers import SearchableText_file
 from plone.namedfile.file import NamedBlobFile
+from Products.CMFPlone.Portal import PloneSite
 
 import logging
 import os
@@ -27,7 +29,7 @@
 
 
 @pytest.fixture
-def java():
+def java() -> None:
     if shutil.which("java") is None:
         if os.environ.get(REQUIRE_JAVA) == "1":
             pytest.fail(f"java is not on PATH and {REQUIRE_JAVA}=1")
@@ -35,13 +37,13 @@ def java():
 
 
 @pytest.fixture
-def no_locale(monkeypatch):
+def no_locale(monkeypatch: pytest.MonkeyPatch) -> None:
     """Run java without a locale, like a slim container image."""
     monkeypatch.setenv("LC_ALL", "C")
     monkeypatch.delenv("LANG", raising=False)
 
 
-def add_file(portal, name: str, content_type: str):
+def add_file(portal: PloneSite, name: str, content_type: str) -> File:
     with api.env.adopt_roles(["Manager"]):
         return api.content.create(
             container=portal,
@@ -56,15 +58,17 @@ def add_file(portal, name: str, content_type: str):
         )
 
 
-def text_of(portal, name: str, content_type: str) -> str:
-    return SearchableText_file(add_file(portal, name, content_type))()
+def text_of(portal: PloneSite, name: str, content_type: str) -> str:
+    text: str = SearchableText_file(add_file(portal, name, content_type))()
+    return text
 
 
-def transformed(portal, name: str, content_type: str) -> str:
+def transformed(portal: PloneSite, name: str, content_type: str) -> str:
     data = portal.portal_transforms.convertTo(
         "text/plain", (SAMPLES / name).read_bytes(), mimetype=content_type
     )
-    return data.getData()
+    text: str = data.getData()
+    return text
 
 
 @pytest.mark.usefixtures("java", "no_locale")
diff --git a/tests/test_setup.py b/tests/test_setup.py
index f6394c7..397b324 100644
--- a/tests/test_setup.py
+++ b/tests/test_setup.py
@@ -1,6 +1,10 @@
 from Acquisition import aq_base
 from c2.transform.msoffice import PACKAGE_NAME
 from plone.base.interfaces import INonInstallable
+from Products.CMFPlone.controlpanel.browser.quickinstaller import InstallerView
+from Products.CMFPlone.Portal import PloneSite
+from Products.GenericSetup.tool import SetupTool
+from Products.PortalTransforms.TransformEngine import TransformTool
 from zope.component import getUtility
 
 import logging
@@ -14,12 +18,12 @@
 
 
 @pytest.fixture
-def transforms(portal):
+def transforms(portal: PloneSite) -> TransformTool:
     return portal.portal_transforms
 
 
 @pytest.fixture
-def setup_tool(portal):
+def setup_tool(portal: PloneSite) -> SetupTool:
     return portal.portal_setup
 
 
@@ -70,7 +74,7 @@ def test_uninstall_profile_is_hidden(portal):
 
 class TestUninstall:
     @pytest.fixture(autouse=True)
-    def uninstalled(self, installer):
+    def uninstalled(self, installer: InstallerView) -> None:
         installer.uninstall_product(PACKAGE_NAME)
 
     def test_addon_uninstalled(self, installer):
diff --git a/tests/test_transform.py b/tests/test_transform.py
index 885fafe..bd21ab2 100644
--- a/tests/test_transform.py
+++ b/tests/test_transform.py
@@ -17,14 +17,22 @@
 class FakeRun:
     """Stands in for subprocess.run and records the calls."""
 
-    def __init__(self, returncode=0, stdout=b"", stderr=b"", error=None):
+    def __init__(
+        self,
+        returncode: int = 0,
+        stdout: bytes = b"",
+        stderr: bytes = b"",
+        error: Exception | None = None,
+    ) -> None:
         self.returncode = returncode
         self.stdout = stdout
         self.stderr = stderr
         self.error = error
-        self.calls = []
+        self.calls: list[tuple[list[str], dict[str, object]]] = []
 
-    def __call__(self, args, **kwargs):
+    def __call__(
+        self, args: list[str], **kwargs: object
+    ) -> subprocess.CompletedProcess[bytes]:
         self.calls.append((args, kwargs))
         if self.error is not None:
             raise self.error
@@ -34,22 +42,30 @@ def __call__(self, args, **kwargs):
 
 
 @pytest.fixture(autouse=True)
-def clean_environment(monkeypatch):
+def clean_environment(monkeypatch: pytest.MonkeyPatch) -> None:
     monkeypatch.delenv("C2_TRANSFORM_MSOFFICE_TIMEOUT", raising=False)
     monkeypatch.delenv("C2_TRANSFORM_MSOFFICE_JAVA_OPTS", raising=False)
 
 
-def fake_run(monkeypatch, **kwargs) -> FakeRun:
-    run = FakeRun(**kwargs)
+def fake_run(
+    monkeypatch: pytest.MonkeyPatch,
+    *,
+    returncode: int = 0,
+    stdout: bytes = b"",
+    stderr: bytes = b"",
+    error: Exception | None = None,
+) -> FakeRun:
+    run = FakeRun(returncode, stdout, stderr, error)
     monkeypatch.setattr(transform.subprocess, "run", run)
     return run
 
 
-def convert(orig) -> str:
+def convert(orig: bytes | str) -> str:
     data = datastream("test")
     result = transform.msoffice_to_text().convert(orig, data)
     assert result is data
-    return data.getData()
+    text: str = data.getData()
+    return text
 
 
 def test_implements_itransform():
diff --git a/tox.ini b/tox.ini
index 05d74b4..67d7aac 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,6 +1,9 @@
 [tox]
+# dependency_groups (the mypy environment) needs tox 4.22
+min_version = 4.22
 envlist =
     ruff-check,
+    mypy,
     py{310,311,312,313}-Plone60,
     py{310,311,312,313}-Plone61,
     py{310,311,312,313,314}-Plone62,
@@ -49,6 +52,17 @@ commands =
     ruff format --check --diff src tests
 
 
+[testenv:mypy]
+basepython = python3.14
+dependency_groups = dev
+setenv =
+    constraints_file=constraints_plone62.txt
+deps =
+    -rrequirements_plone62.txt
+commands =
+    mypy src tests
+
+
 [testenv:ruff-fix]
 basepython = python3.12
 skip_install = True