diff --git a/.gitignore b/.gitignore
--- a/.gitignore
+++ b/.gitignore
@@ -12,4 +12,5 @@
 /sql/createdb-stamp
 /sql/filldb-stamp
 .tox/
-.hypothesis/
\ No newline at end of file
+.hypothesis/
+.mypy_cache/
diff --git a/MANIFEST.in b/MANIFEST.in
--- a/MANIFEST.in
+++ b/MANIFEST.in
@@ -6,3 +6,4 @@
 recursive-include sql *
 recursive-include swh/indexer/sql *.sql
 recursive-include swh/indexer/data *
+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-magic.*]
+ignore_missing_imports = True
+
+[mypy-pkg_resources.*]
+ignore_missing_imports = True
+
+[mypy-psycopg2.*]
+ignore_missing_imports = True
+
+[mypy-pyld.*]
+ignore_missing_imports = True
+
+[mypy-pytest.*]
+ignore_missing_imports = True
+
+[mypy-xmltodict.*]
+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/indexer/cli.py b/swh/indexer/cli.py
--- a/swh/indexer/cli.py
+++ b/swh/indexer/cli.py
@@ -238,7 +238,7 @@
 
 
 @cli.command('rpc-serve')
-@click.argument('config-path', required=1)
+@click.argument('config-path', required=True)
 @click.option('--host', default='0.0.0.0', help="Host to run the server")
 @click.option('--port', default=5007, type=click.INT,
               help="Binding port of the server")
diff --git a/swh/indexer/fossology_license.py b/swh/indexer/fossology_license.py
--- a/swh/indexer/fossology_license.py
+++ b/swh/indexer/fossology_license.py
@@ -5,8 +5,9 @@
 
 import subprocess
 
-from swh.model import hashutil
+from typing import Optional
 
+from swh.model import hashutil
 from .indexer import ContentIndexer, ContentRangeIndexer, write_to_temp
 
 
@@ -66,7 +67,7 @@
         'write_batch_size': ('int', 1000),
     }
 
-    CONFIG_BASE_FILENAME = 'indexer/fossology_license'
+    CONFIG_BASE_FILENAME = 'indexer/fossology_license'  # type: Optional[str]
 
     def prepare(self):
         super().prepare()
diff --git a/swh/indexer/indexer.py b/swh/indexer/indexer.py
--- a/swh/indexer/indexer.py
+++ b/swh/indexer/indexer.py
@@ -9,8 +9,10 @@
 import shutil
 import tempfile
 import datetime
+
 from copy import deepcopy
 from contextlib import contextmanager
+from typing import Any, Dict, Tuple
 
 from swh.scheduler import get_scheduler
 from swh.scheduler import CONFIG as SWH_CONFIG
@@ -124,7 +126,7 @@
         })
     }
 
-    ADDITIONAL_CONFIG = {}
+    ADDITIONAL_CONFIG = {}  # type: Dict[str, Tuple[str, Any]]
 
     USE_TOOLS = True
 
diff --git a/swh/indexer/metadata_dictionary/base.py b/swh/indexer/metadata_dictionary/base.py
--- a/swh/indexer/metadata_dictionary/base.py
+++ b/swh/indexer/metadata_dictionary/base.py
@@ -7,6 +7,8 @@
 import json
 import logging
 
+from typing import List
+
 from swh.indexer.codemeta import SCHEMA_URI
 from swh.indexer.codemeta import compact
 
@@ -109,7 +111,7 @@
     """Base class for mappings that take as input a file that is mostly
     a key-value store (eg. a shallow JSON dict)."""
 
-    string_fields = []
+    string_fields = []  # type: List[str]
     '''List of fields that are simple strings, and don't need any
     normalization.'''
 
diff --git a/swh/indexer/mimetype.py b/swh/indexer/mimetype.py
--- a/swh/indexer/mimetype.py
+++ b/swh/indexer/mimetype.py
@@ -5,6 +5,8 @@
 
 import magic
 
+from typing import Optional
+
 from .indexer import ContentIndexer, ContentRangeIndexer
 
 if not hasattr(magic.Magic, 'from_buffer'):
@@ -51,7 +53,7 @@
         'write_batch_size': ('int', 1000),
     }
 
-    CONFIG_BASE_FILENAME = 'indexer/mimetype'
+    CONFIG_BASE_FILENAME = 'indexer/mimetype'  # type: Optional[str]
 
     def index(self, id, data):
         """Index sha1s' content and store result.
diff --git a/swh/indexer/py.typed b/swh/indexer/py.typed
new file mode 100644
--- /dev/null
+++ b/swh/indexer/py.typed
@@ -0,0 +1 @@
+# Marker file for PEP 561.
diff --git a/swh/indexer/tests/test_metadata.py b/swh/indexer/tests/test_metadata.py
--- a/swh/indexer/tests/test_metadata.py
+++ b/swh/indexer/tests/test_metadata.py
@@ -7,6 +7,7 @@
 import unittest
 
 from hypothesis import given, strategies, settings, HealthCheck
+from typing import cast
 
 from swh.model.hashutil import hash_to_bytes
 
@@ -14,6 +15,9 @@
 from swh.indexer.codemeta import merge_documents
 from swh.indexer.metadata_dictionary import MAPPINGS
 from swh.indexer.metadata_dictionary.base import merge_values
+from swh.indexer.metadata_dictionary.maven import MavenMapping
+from swh.indexer.metadata_dictionary.npm import NpmMapping
+from swh.indexer.metadata_dictionary.ruby import GemspecMapping
 from swh.indexer.metadata_detector import (
     detect_metadata
 )
@@ -1075,7 +1079,7 @@
 
     @settings(suppress_health_check=[HealthCheck.too_slow])
     @given(json_document_strategy(
-        keys=list(MAPPINGS['NpmMapping'].mapping)))
+        keys=list(cast(NpmMapping, MAPPINGS['NpmMapping']).mapping)))
     def test_npm_adversarial(self, doc):
         raw = json.dumps(doc).encode()
         self.npm_mapping.translate(raw)
@@ -1088,7 +1092,7 @@
 
     @settings(suppress_health_check=[HealthCheck.too_slow])
     @given(xml_document_strategy(
-        keys=list(MAPPINGS['MavenMapping'].mapping),
+        keys=list(cast(MavenMapping, MAPPINGS['MavenMapping']).mapping),
         root='project',
         xmlns='http://maven.apache.org/POM/4.0.0'))
     def test_maven_adversarial(self, doc):
@@ -1099,7 +1103,8 @@
         # keys
         strategies.one_of(
             strategies.text(),
-            *map(strategies.just, MAPPINGS['GemspecMapping'].mapping)
+            *map(strategies.just,
+                 cast(GemspecMapping, MAPPINGS['GemspecMapping']).mapping)
         ),
         # values
         strategies.recursive(
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 =
@@ -39,3 +39,11 @@
   flake8
 commands =
   {envpython} -m flake8
+
+[testenv:mypy]
+skip_install = true
+deps =
+  .[testing]
+  mypy
+commands =
+  mypy swh