diff --git a/.gitignore b/.gitignore
--- a/.gitignore
+++ b/.gitignore
@@ -10,3 +10,4 @@
 dist/
 version.txt
 .tox/
+.mypy_cache/
diff --git a/MANIFEST.in b/MANIFEST.in
--- a/MANIFEST.in
+++ b/MANIFEST.in
@@ -4,3 +4,4 @@
 include version.txt
 include README.md
 recursive-include swh/loader/package/tests/ *.tar.gz
+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-celery.*]
+ignore_missing_imports = True
+
+[mypy-pkg_resources.*]
+ignore_missing_imports = True
+
+[mypy-psutil.*]
+ignore_missing_imports = True
+
+[mypy-psycopg2.*]
+ignore_missing_imports = True
+
+[mypy-pytest.*]
+ignore_missing_imports = True
+
+[mypy-requests_mock.*]
+ignore_missing_imports = True
+
+[mypy-retrying.*]
+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/core/loader.py b/swh/loader/core/loader.py
--- a/swh/loader/core/loader.py
+++ b/swh/loader/core/loader.py
@@ -14,6 +14,7 @@
 
 from abc import ABCMeta, abstractmethod
 from retrying import retry
+from typing import Any, Dict, Optional, Tuple
 
 from . import converters
 
@@ -112,7 +113,7 @@
     - :class:`DebianLoader`
 
     """
-    CONFIG_BASE_FILENAME = None
+    CONFIG_BASE_FILENAME = None  # type: Optional[str]
 
     DEFAULT_CONFIG = {
         'storage': ('dict', {
@@ -139,9 +140,9 @@
         'revision_packet_size': ('int', 100000),
         'release_packet_size': ('int', 100000),
         'occurrence_packet_size': ('int', 100000),
-    }
+    }  # type: Dict[str, Tuple[str, Any]]
 
-    ADDITIONAL_CONFIG = {}
+    ADDITIONAL_CONFIG = {}  # type: Dict[str, Tuple[str, Any]]
 
     def __init__(self, logging_class=None, config=None):
         if config:
@@ -906,7 +907,7 @@
     inherit directly from :class:`BufferedLoader`.
 
     """
-    ADDITIONAL_CONFIG = {}
+    ADDITIONAL_CONFIG = {}  # type: Dict[str, Tuple[str, Any]]
 
     def __init__(self, logging_class=None, config=None):
         super().__init__(logging_class=logging_class, config=config)
diff --git a/swh/loader/core/py.typed b/swh/loader/core/py.typed
new file mode 100644
--- /dev/null
+++ b/swh/loader/core/py.typed
@@ -0,0 +1 @@
+# Marker file for PEP 561.
diff --git a/swh/loader/core/tests/test_loader.py b/swh/loader/core/tests/test_loader.py
--- a/swh/loader/core/tests/test_loader.py
+++ b/swh/loader/core/tests/test_loader.py
@@ -19,7 +19,7 @@
     def cleanup(self):
         pass
 
-    def prepare(self):
+    def prepare(self, *args, **kwargs):
         pass
 
     def fetch_data(self):
@@ -28,7 +28,7 @@
     def store_data(self):
         pass
 
-    def prepare_origin_visit(self):
+    def prepare_origin_visit(self, *args, **kwargs):
         origin = self.storage.origin_get(
             self._test_prepare_origin_visit_data['origin'])
         self.origin = origin
diff --git a/swh/loader/package/loader.py b/swh/loader/package/loader.py
--- a/swh/loader/package/loader.py
+++ b/swh/loader/package/loader.py
@@ -8,7 +8,7 @@
 import requests
 
 try:
-    from _version import __version__
+    from _version import __version__  # type: ignore
 except ImportError:
     __version__ = 'devel'
 
diff --git a/swh/loader/package/tests/test_loader.py b/swh/loader/package/tests/test_loader.py
--- a/swh/loader/package/tests/test_loader.py
+++ b/swh/loader/package/tests/test_loader.py
@@ -6,6 +6,9 @@
 import unittest
 import os
 import requests_mock
+
+from typing import List
+
 from swh.loader.package.loader import GNULoader
 from swh.loader.core.tests import BaseLoaderStorageTest
 from swh.loader.package.tests.common import (
@@ -98,8 +101,8 @@
     }
     _expected_new_snapshot_first_visit = '2ae491bbaeef7351641997d1b9193aa2a67d26bc' # noqa
 
-    _expected_new_contents_invalid_origin = []
-    _expected_new_directories_invalid_origin = []
+    _expected_new_contents_invalid_origin = []  # type: List[str]
+    _expected_new_directories_invalid_origin = []  # type: List[str]
 
     @classmethod
     def setUpClass(cls):
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 =
@@ -14,3 +14,11 @@
   flake8
 commands =
   {envpython} -m flake8
+
+[testenv:mypy]
+skip_install = true
+deps =
+  .[testing]
+  mypy
+commands =
+  mypy swh