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
@@ -2,3 +2,4 @@
 include requirements.txt
 include requirements-swh.txt
 include version.txt
+recursive-include swh py.typed
diff --git a/mypy.ini b/mypy.ini
new file mode 100644
--- /dev/null
+++ b/mypy.ini
@@ -0,0 +1,21 @@
+[mypy]
+namespace_packages = True
+warn_unused_ignores = True
+
+
+# 3rd party libraries without stubs (yet)
+
+[mypy-azure.*]
+ignore_missing_imports = True
+
+[mypy-libcloud.*]
+ignore_missing_imports = True
+
+[mypy-pkg_resources.*]
+ignore_missing_imports = True
+
+[mypy-pytest.*]
+ignore_missing_imports = True
+
+[mypy-rados.*]
+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 typing import Iterable
+
+__path__ = __import__('pkgutil').extend_path(__path__,
+                                             __name__)  # type: Iterable[str]
diff --git a/swh/objstorage/__init__.py b/swh/objstorage/__init__.py
--- a/swh/objstorage/__init__.py
+++ b/swh/objstorage/__init__.py
@@ -13,6 +13,7 @@
 from swh.objstorage.backends.seaweed import WeedObjStorage
 from swh.objstorage.backends.generator import RandomGeneratorObjStorage
 
+from typing import Callable, Dict, Union
 
 __all__ = ['get_objstorage', 'ObjStorage']
 
@@ -23,7 +24,7 @@
     'memory': InMemoryObjStorage,
     'weed': WeedObjStorage,
     'random': RandomGeneratorObjStorage,
-}
+}  # type: Dict[str, Union[type, Callable[..., type]]]
 
 _STORAGE_CLASSES_MISSING = {
 }
diff --git a/swh/objstorage/api/server.py b/swh/objstorage/api/server.py
--- a/swh/objstorage/api/server.py
+++ b/swh/objstorage/api/server.py
@@ -6,6 +6,7 @@
 import os
 
 import aiohttp.web
+import json
 
 from swh.core.config import read as config_read
 from swh.core.api.asynchronous import (RPCServerApp, decode_request,
diff --git a/swh/objstorage/py.typed b/swh/objstorage/py.typed
new file mode 100644
--- /dev/null
+++ b/swh/objstorage/py.typed
@@ -0,0 +1 @@
+# Marker file for PEP 561.
diff --git a/swh/objstorage/tests/test_objstorage_azure.py b/swh/objstorage/tests/test_objstorage_azure.py
--- a/swh/objstorage/tests/test_objstorage_azure.py
+++ b/swh/objstorage/tests/test_objstorage_azure.py
@@ -7,6 +7,8 @@
 from collections import defaultdict
 from unittest.mock import patch
 
+from typing import Any, Dict
+
 from azure.common import AzureMissingResourceHttpError
 from swh.model.hashutil import hash_to_hex
 from swh.objstorage import get_objstorage
@@ -25,40 +27,40 @@
     """Mock internal azure library which AzureCloudObjStorage depends upon.
 
     """
-    data = {}
+    _data = {}  # type: Dict[str, Any]
 
     def __init__(self, account_name, account_key, **kwargs):
         # do not care for the account_name and the api_secret_key here
-        self.data = defaultdict(dict)
+        self._data = defaultdict(dict)
 
     def get_container_properties(self, container_name):
-        self.data[container_name]
-        return container_name in self.data
+        self._data[container_name]
+        return container_name in self._data
 
     def create_blob_from_bytes(self, container_name, blob_name, blob):
-        self.data[container_name][blob_name] = blob
+        self._data[container_name][blob_name] = blob
 
     def get_blob_to_bytes(self, container_name, blob_name):
-        if blob_name not in self.data[container_name]:
+        if blob_name not in self._data[container_name]:
             raise AzureMissingResourceHttpError(
                 'Blob %s not found' % blob_name,
                 404)
         return MockBlob(name=blob_name,
-                        content=self.data[container_name][blob_name])
+                        content=self._data[container_name][blob_name])
 
     def delete_blob(self, container_name, blob_name):
         try:
-            self.data[container_name].pop(blob_name)
+            self._data[container_name].pop(blob_name)
         except KeyError:
             raise AzureMissingResourceHttpError(
                 'Blob %s not found' % blob_name, 404)
         return True
 
     def exists(self, container_name, blob_name):
-        return blob_name in self.data[container_name]
+        return blob_name in self._data[container_name]
 
     def list_blobs(self, container_name, marker=None, maxresults=None):
-        for blob_name, content in sorted(self.data[container_name].items()):
+        for blob_name, content in sorted(self._data[container_name].items()):
             if marker is None or blob_name > marker:
                 yield MockBlob(name=blob_name, content=content)
 
diff --git a/tox.ini b/tox.ini
--- a/tox.ini
+++ b/tox.ini
@@ -1,5 +1,5 @@
 [tox]
-envlist=flake8,py3
+envlist=flake8,py3,mypy
 
 [testenv]
 deps =
@@ -14,3 +14,11 @@
   flake8
 commands =
   {envpython} -m flake8
+
+[testenv:mypy]
+skip_install = true
+deps =
+  .[testing]
+  mypy
+commands =
+  mypy swh