diff --git a/.gitignore b/.gitignore
--- a/.gitignore
+++ b/.gitignore
@@ -10,3 +10,4 @@
 build/
 dist/
 .tox/
+.mypy_cache/
diff --git a/MANIFEST.in b/MANIFEST.in
--- a/MANIFEST.in
+++ b/MANIFEST.in
@@ -3,3 +3,4 @@
 include requirements-swh.txt
 include version.txt
 recursive-include swh/loader/mercurial/tests/resources *
+recursive-include swh py.typed
diff --git a/mypy.ini b/mypy.ini
new file mode 100644
--- /dev/null
+++ b/mypy.ini
@@ -0,0 +1,27 @@
+[mypy]
+namespace_packages = True
+warn_unused_ignores = True
+
+
+# 3rd party libraries without stubs (yet)
+
+[mypy-billiard.*]
+ignore_missing_imports = True
+
+[mypy-celery.*]
+ignore_missing_imports = True
+
+[mypy-hglib.*]
+ignore_missing_imports = True
+
+[mypy-pkg_resources.*]
+ignore_missing_imports = True
+
+[mypy-patoolib.*]
+ignore_missing_imports = True
+
+[mypy-pytest.*]
+ignore_missing_imports = True
+
+[mypy-sqlitedict.*]
+ignore_missing_imports = True
diff --git a/swh/__init__.py b/swh/__init__.py
--- a/swh/__init__.py
+++ b/swh/__init__.py
@@ -1 +1,4 @@
-__path__ = __import__('pkgutil').extend_path(__path__, __name__)
+from pkgutil import extend_path
+from typing import Iterable
+
+__path__ = extend_path(__path__, __name__)  # type: Iterable[str]
diff --git a/swh/loader/__init__.py b/swh/loader/__init__.py
--- a/swh/loader/__init__.py
+++ b/swh/loader/__init__.py
@@ -1 +1,4 @@
-__path__ = __import__('pkgutil').extend_path(__path__, __name__)
+from pkgutil import extend_path
+from typing import Iterable
+
+__path__ = extend_path(__path__, __name__)  # type: Iterable[str]
diff --git a/swh/loader/mercurial/py.typed b/swh/loader/mercurial/py.typed
new file mode 100644
--- /dev/null
+++ b/swh/loader/mercurial/py.typed
@@ -0,0 +1 @@
+# Marker file for PEP 561.
diff --git a/swh/loader/mercurial/tests/conftest.py b/swh/loader/mercurial/tests/conftest.py
--- a/swh/loader/mercurial/tests/conftest.py
+++ b/swh/loader/mercurial/tests/conftest.py
@@ -3,7 +3,7 @@
 from swh.scheduler.tests.conftest import *  # noqa
 
 
-@pytest.fixture(scope='session')
+@pytest.fixture(scope='session')  # type: ignore  # expected redefinition
 def celery_includes():
     return [
         'swh.loader.mercurial.tasks',
diff --git a/tox.ini b/tox.ini
--- a/tox.ini
+++ b/tox.ini
@@ -1,5 +1,5 @@
 [tox]
-envlist=flake8,py3
+envlist=flake8,mypy,py3
 
 [testenv:py3]
 deps =
@@ -16,3 +16,11 @@
   flake8
 commands =
   {envpython} -m flake8
+
+[testenv:mypy]
+skip_install = true
+deps =
+  .[testing]
+  mypy
+commands =
+  mypy swh