diff --git a/.github/workflows/cibuild.yml b/.github/workflows/cibuild.yml
index 0e44c4b..9b81e30 100644
--- a/.github/workflows/cibuild.yml
+++ b/.github/workflows/cibuild.yml
@@ -33,6 +33,15 @@ jobs:
     - run: pip install tox
     - run: tox -e flake8
 
+  mypy:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v1
+    - uses: actions/setup-python@v2
+    - run: python -m pip install --upgrade pip
+    - run: pip install tox
+    - run: tox -e mypy
+
   unit_tests:
     strategy:
       fail-fast: false
diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
index 2708aa6..e596b1e 100644
--- a/.pre-commit-config.yaml
+++ b/.pre-commit-config.yaml
@@ -21,3 +21,9 @@ repos:
       args: [--config=.flake8]
       language: system
       files: \.py$
+    - id: mypy
+      name: mypy
+      entry: mypy
+      stages: [commit]
+      language: system
+      files: \.py$
diff --git a/pyproject.toml b/pyproject.toml
index 15a19d1..1c56279 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -18,3 +18,8 @@ line_length = 119
 multi_line_output = 3
 use_parentheses = true
 include_trailing_comma = true
+
+
+[tool.mypy]
+exclude = "thirdparty"
+
diff --git a/seleniumwire/__main__.py b/seleniumwire/__main__.py
index 82c17e0..9144442 100644
--- a/seleniumwire/__main__.py
+++ b/seleniumwire/__main__.py
@@ -2,6 +2,7 @@
 import logging
 import signal
 from argparse import RawDescriptionHelpFormatter
+from typing import Callable, Dict
 
 from seleniumwire import backend, utils
 
@@ -23,10 +24,13 @@ def standalone_proxy(port=0, addr='127.0.0.1'):
     signal.signal(signal.SIGINT, lambda *_: b.shutdown())
 
 
+# Mapping of command names to the command callables
+COMMANDS: Dict[str, Callable] = {'extractcert': utils.extract_cert, 'standaloneproxy': standalone_proxy}
+
+
 if __name__ == '__main__':
-    commands = {'extractcert': utils.extract_cert, 'standaloneproxy': standalone_proxy}
     parser = argparse.ArgumentParser(
-        description='\n\nsupported commands: \n  %s' % '\n  '.join(sorted(commands)),
+        description='\n\nsupported commands: \n  %s' % '\n  '.join(sorted(COMMANDS)),
         formatter_class=RawDescriptionHelpFormatter,
         usage='python -m seleniumwire <command>',
     )
@@ -40,10 +44,10 @@ def standalone_proxy(port=0, addr='127.0.0.1'):
 
     args = parser.parse_args()
     pargs = [arg for arg in args.args if '=' not in arg and arg is not args.command]
-    kwargs = dict([tuple(arg.split('=')) for arg in args.args if '=' in arg])
+    kwargs: Dict[str, str] = dict([arg.split('=') for arg in args.args if '=' in arg])
 
     try:
-        commands[args.command](*pargs, **kwargs)
+        COMMANDS[args.command](*pargs, **kwargs)
     except KeyError:
         print("Unsupported command '{}' (use --help for list of commands)".format(args.command))
     except TypeError as e:
diff --git a/seleniumwire/inspect.py b/seleniumwire/inspect.py
index 7e03f89..6dc3dcc 100644
--- a/seleniumwire/inspect.py
+++ b/seleniumwire/inspect.py
@@ -1,8 +1,8 @@
 import inspect
 import time
-from typing import Iterator, List, Optional, Union
+from typing import Callable, Iterator, List, Optional, Union
 
-from selenium.common.exceptions import TimeoutException
+from selenium.common.exceptions import TimeoutException  # type: ignore
 
 from seleniumwire import har
 from seleniumwire.request import Request
@@ -23,18 +23,18 @@ def requests(self) -> List[Request]:
             A list of Request instances representing the requests made
             between the browser and server.
         """
-        return self.backend.storage.load_requests()
+        return self.backend.storage.load_requests()  # type: ignore
 
     @requests.deleter
     def requests(self):
-        self.backend.storage.clear_requests()
+        self.backend.storage.clear_requests()  # type: ignore
 
     def iter_requests(self) -> Iterator[Request]:
         """Return an iterator of requests.
 
         Returns: An iterator.
         """
-        yield from self.backend.storage.iter_requests()
+        yield from self.backend.storage.iter_requests()  # type: ignore
 
     @property
     def last_request(self) -> Optional[Request]:
@@ -46,7 +46,7 @@ def last_request(self) -> Optional[Request]:
             A Request instance representing the last request made, or
             None if no requests have been made.
         """
-        return self.backend.storage.load_last_request()
+        return self.backend.storage.load_last_request()  # type: ignore
 
     def wait_for_request(self, pat: str, timeout: Union[int, float] = 10) -> Request:
         """Wait up to the timeout period for a request matching the specified
@@ -73,7 +73,7 @@ def wait_for_request(self, pat: str, timeout: Union[int, float] = 10) -> Request
         start = time.time()
 
         while time.time() - start < timeout:
-            request = self.backend.storage.find(pat)
+            request = self.backend.storage.find(pat)  # type: ignore
 
             if request is None:
                 time.sleep(1 / 5)
@@ -91,7 +91,7 @@ def har(self) -> str:
 
         Returns: A JSON string of HAR data.
         """
-        return har.generate_har(self.backend.storage.load_har_entries())
+        return har.generate_har(self.backend.storage.load_har_entries())  # type: ignore
 
     @property
     def header_overrides(self):
@@ -119,7 +119,7 @@ def header_overrides(self):
                 ('*.somewhere-else.com.*', {'User-Agent': 'Chrome'})
             ]
         """
-        return self.backend.modifier.headers
+        return self.backend.modifier.headers  # type: ignore
 
     @header_overrides.setter
     def header_overrides(self, headers):
@@ -129,16 +129,16 @@ def header_overrides(self, headers):
         else:
             self._validate_headers(headers)
 
-        self.backend.modifier.headers = headers
+        self.backend.modifier.headers = headers  # type: ignore
 
     def _validate_headers(self, headers):
         for v in headers.values():
             if v is not None:
                 assert isinstance(v, str), 'Header values must be strings'
 
-    @header_overrides.deleter
+    @header_overrides.deleter  # type: ignore
     def header_overrides(self):
-        del self.backend.modifier.headers
+        del self.backend.modifier.headers  # type: ignore
 
     @property
     def param_overrides(self):
@@ -164,15 +164,15 @@ def param_overrides(self):
                 ('*.somewhere-else.com.*', {'x': 'y'}),
             ]
         """
-        return self.backend.modifier.params
+        return self.backend.modifier.params  # type: ignore
 
     @param_overrides.setter
     def param_overrides(self, params):
-        self.backend.modifier.params = params
+        self.backend.modifier.params = params  # type: ignore
 
     @param_overrides.deleter
     def param_overrides(self):
-        del self.backend.modifier.params
+        del self.backend.modifier.params  # type: ignore
 
     @property
     def body_overrides(self):
@@ -194,15 +194,15 @@ def body_overrides(self):
                 ('*.somewhere-else.com.*', '{"x":"y"}'),
             ]
         """
-        return self.backend.modifier.bodies
+        return self.backend.modifier.bodies  # type: ignore
 
     @body_overrides.setter
     def body_overrides(self, bodies):
-        self.backend.modifier.bodies = bodies
+        self.backend.modifier.bodies = bodies  # type: ignore
 
     @body_overrides.deleter
     def body_overrides(self):
-        del self.backend.modifier.bodies
+        del self.backend.modifier.bodies  # type: ignore
 
     @property
     def querystring_overrides(self):
@@ -223,15 +223,15 @@ def querystring_overrides(self):
                 ('*.somewhere-else.com.*', 'a=b&c=d'),
             ]
         """
-        return self.backend.modifier.querystring
+        return self.backend.modifier.querystring  # type: ignore
 
     @querystring_overrides.setter
     def querystring_overrides(self, querystrings):
-        self.backend.modifier.querystring = querystrings
+        self.backend.modifier.querystring = querystrings  # type: ignore
 
     @querystring_overrides.deleter
     def querystring_overrides(self):
-        del self.backend.modifier.querystring
+        del self.backend.modifier.querystring  # type: ignore
 
     @property
     def rewrite_rules(self):
@@ -248,15 +248,15 @@ def rewrite_rules(self):
                 (r'https://docs.python.org/2/', r'https://docs.python.org/3/'),
             ]
         """
-        return self.backend.modifier.rewrite_rules
+        return self.backend.modifier.rewrite_rules  # type: ignore
 
     @rewrite_rules.setter
     def rewrite_rules(self, rewrite_rules):
-        self.backend.modifier.rewrite_rules = rewrite_rules
+        self.backend.modifier.rewrite_rules = rewrite_rules  # type: ignore
 
     @rewrite_rules.deleter
     def rewrite_rules(self):
-        del self.backend.modifier.rewrite_rules
+        del self.backend.modifier.rewrite_rules  # type: ignore
 
     @property
     def scopes(self) -> List[str]:
@@ -271,48 +271,48 @@ def scopes(self) -> List[str]:
                 '.*github.*'
             ]
         """
-        return self.backend.scopes
+        return self.backend.scopes  # type: ignore
 
     @scopes.setter
     def scopes(self, scopes: List[str]):
-        self.backend.scopes = scopes
+        self.backend.scopes = scopes  # type: ignore
 
     @scopes.deleter
     def scopes(self):
-        self.backend.scopes = []
+        self.backend.scopes = []  # type: ignore
 
     @property
-    def request_interceptor(self) -> callable:
+    def request_interceptor(self) -> Callable:
         """A callable that will be used to intercept/modify requests.
 
         The callable must accept a single argument for the request
         being intercepted.
         """
-        return self.backend.request_interceptor
+        return self.backend.request_interceptor  # type: ignore
 
     @request_interceptor.setter
-    def request_interceptor(self, interceptor: callable):
-        self.backend.request_interceptor = interceptor
+    def request_interceptor(self, interceptor: Callable):
+        self.backend.request_interceptor = interceptor  # type: ignore
 
     @request_interceptor.deleter
     def request_interceptor(self):
-        self.backend.request_interceptor = None
+        self.backend.request_interceptor = None  # type: ignore
 
     @property
-    def response_interceptor(self) -> callable:
+    def response_interceptor(self) -> Callable:
         """A callable that will be used to intercept/modify responses.
 
         The callable must accept two arguments: the response being
         intercepted and the originating request.
         """
-        return self.backend.response_interceptor
+        return self.backend.response_interceptor  # type: ignore
 
     @response_interceptor.setter
-    def response_interceptor(self, interceptor: callable):
+    def response_interceptor(self, interceptor: Callable):
         if len(inspect.signature(interceptor).parameters) != 2:
             raise RuntimeError('A response interceptor takes two parameters: the request and response')
-        self.backend.response_interceptor = interceptor
+        self.backend.response_interceptor = interceptor  # type: ignore
 
     @response_interceptor.deleter
     def response_interceptor(self):
-        self.backend.response_interceptor = None
+        self.backend.response_interceptor = None  # type: ignore
diff --git a/seleniumwire/request.py b/seleniumwire/request.py
index 7d640a7..84a8510 100644
--- a/seleniumwire/request.py
+++ b/seleniumwire/request.py
@@ -19,6 +19,8 @@ def __repr__(self):
 class Request:
     """Represents an HTTP request."""
 
+    _body: bytes
+
     def __init__(self, *, method: str, url: str, headers: Iterable[Tuple[str, str]], body: bytes = b''):
         """Initialise a new Request object.
 
@@ -119,7 +121,7 @@ def host(self) -> str:
         """
         return urlsplit(self.url).netloc
 
-    @path.setter
+    @path.setter  # type: ignore
     def path(self, p: str):
         parts = list(urlsplit(self.url))
         parts[2] = p
diff --git a/seleniumwire/undetected_chromedriver/__init__.py b/seleniumwire/undetected_chromedriver/__init__.py
index a470664..dce2efd 100644
--- a/seleniumwire/undetected_chromedriver/__init__.py
+++ b/seleniumwire/undetected_chromedriver/__init__.py
@@ -1,5 +1,5 @@
 try:
-    import undetected_chromedriver as uc
+    import undetected_chromedriver as uc  # type: ignore
 except ImportError as e:
     raise ImportError(
         'undetected_chromedriver not found. ' 'Install it with `pip install undetected_chromedriver`.'
@@ -8,5 +8,5 @@
 from seleniumwire.webdriver import Chrome
 
 uc._Chrome = Chrome
-Chrome = uc.Chrome
+Chrome = uc.Chrome  # type: ignore
 ChromeOptions = uc.ChromeOptions  # noqa: F811
diff --git a/seleniumwire/undetected_chromedriver/v2.py b/seleniumwire/undetected_chromedriver/v2.py
index 24bfdaa..6604a5d 100644
--- a/seleniumwire/undetected_chromedriver/v2.py
+++ b/seleniumwire/undetected_chromedriver/v2.py
@@ -1,7 +1,7 @@
 import logging
 
-import undetected_chromedriver.v2 as uc
-from selenium.webdriver import DesiredCapabilities
+import undetected_chromedriver.v2 as uc  # type: ignore
+from selenium.webdriver import DesiredCapabilities  # type: ignore
 
 from seleniumwire import backend
 from seleniumwire.inspect import InspectRequestsMixin
diff --git a/seleniumwire/utils.py b/seleniumwire/utils.py
index aa091b7..ac3ec44 100644
--- a/seleniumwire/utils.py
+++ b/seleniumwire/utils.py
@@ -2,10 +2,9 @@
 import logging
 import os
 import pkgutil
-from collections import namedtuple
 from pathlib import Path
-from typing import Dict, NamedTuple
-from urllib.request import _parse_proxy
+from typing import Dict, List, Sequence, Union
+from urllib.request import _parse_proxy  # type: ignore
 
 from seleniumwire.thirdparty.mitmproxy.net.http import encoding as decoder
 
@@ -16,7 +15,7 @@
 COMBINED_CERT = 'seleniumwire-ca.pem'
 
 
-def get_upstream_proxy(options):
+def get_upstream_proxy(options: Dict[str, Union[str, Dict]]) -> Dict[str, str]:
     """Get the upstream proxy configuration from the options dictionary.
     This will be overridden with any configuration found in the environment
     variables HTTP_PROXY, HTTPS_PROXY, NO_PROXY
@@ -34,13 +33,16 @@ def get_upstream_proxy(options):
         options: The selenium wire options.
     Returns: A dictionary.
     """
-    proxy_options = (options or {}).pop('proxy', {})
+    try:
+        proxy_options: Dict[str, str] = options['proxy']  # type: ignore
+    except KeyError:
+        proxy_options = {}
 
     http_proxy = os.environ.get('HTTP_PROXY')
     https_proxy = os.environ.get('HTTPS_PROXY')
     no_proxy = os.environ.get('NO_PROXY')
 
-    merged = {}
+    merged: Dict[str, str] = {}
 
     if http_proxy:
         merged['http'] = http_proxy
@@ -51,22 +53,10 @@ def get_upstream_proxy(options):
 
     merged.update(proxy_options)
 
-    no_proxy = merged.get('no_proxy')
-    if isinstance(no_proxy, str):
-        merged['no_proxy'] = [h.strip() for h in no_proxy.split(',')]
-
-    conf = namedtuple('ProxyConf', 'scheme username password hostport')
-
-    for proxy_type in ('http', 'https'):
-        # Parse the upstream proxy URL into (scheme, username, password, hostport)
-        # for ease of access.
-        if merged.get(proxy_type) is not None:
-            merged[proxy_type] = conf(*_parse_proxy(merged[proxy_type]))
-
     return merged
 
 
-def build_proxy_args(proxy_config: Dict[str, NamedTuple]) -> Dict[str, str]:
+def build_proxy_args(proxy_config: Dict[str, str]) -> Dict[str, Union[str, List[str]]]:
     """Build the arguments needed to pass an upstream proxy to mitmproxy.
 
     Args:
@@ -77,21 +67,15 @@ def build_proxy_args(proxy_config: Dict[str, NamedTuple]) -> Dict[str, str]:
     https_proxy = proxy_config.get('https')
     conf = None
 
-    if http_proxy and https_proxy:
-        if http_proxy.hostport != https_proxy.hostport:  # noqa
-            # We only support a single upstream proxy server
-            raise ValueError('Different settings for http and https proxy servers not supported')
-
+    if https_proxy:
         conf = https_proxy
     elif http_proxy:
         conf = http_proxy
-    elif https_proxy:
-        conf = https_proxy
 
-    args = {}
+    args: Dict[str, Union[str, List[str]]] = {}
 
     if conf:
-        scheme, username, password, hostport = conf
+        scheme, username, password, hostport = _parse_proxy(conf)
 
         args['mode'] = 'upstream:{}://{}'.format(scheme, hostport)
 
@@ -106,17 +90,17 @@ def build_proxy_args(proxy_config: Dict[str, NamedTuple]) -> Dict[str, str]:
         no_proxy = proxy_config.get('no_proxy')
 
         if no_proxy:
-            args['no_proxy'] = no_proxy
+            args['no_proxy'] = [h.strip() for h in no_proxy.split(',')]
 
     return args
 
 
-def extract_cert(cert_name='ca.crt'):
+def extract_cert(cert_name: str = 'ca.crt') -> None:
     """Extracts the root certificate to the current working directory."""
 
-    try:
-        cert = pkgutil.get_data(__package__, cert_name)
-    except FileNotFoundError:
+    cert = pkgutil.get_data(__package__, cert_name)
+
+    if cert is None:
         log.error("Invalid certificate '{}'".format(cert_name))
     else:
         with open(Path(os.getcwd(), cert_name), 'wb') as out:
@@ -124,7 +108,7 @@ def extract_cert(cert_name='ca.crt'):
         log.info('{} extracted. You can now import this into a browser.'.format(cert_name))
 
 
-def extract_cert_and_key(dest_folder, check_exists=True):
+def extract_cert_and_key(dest_folder: Union[str, Path], check_exists: bool = True) -> None:
     """Extracts the root certificate and key and combines them into a
     single file called seleniumwire-ca.pem in the specified destination
     folder.
@@ -143,11 +127,14 @@ def extract_cert_and_key(dest_folder, check_exists=True):
     root_cert = pkgutil.get_data(__package__, ROOT_CERT)
     root_key = pkgutil.get_data(__package__, ROOT_KEY)
 
-    with open(combined_path, 'wb') as f_out:
-        f_out.write(root_cert + root_key)
+    if root_cert is None or root_key is None:
+        log.error('Root certificate and/or key missing')
+    else:
+        with open(combined_path, 'wb') as f_out:
+            f_out.write(root_cert + root_key)
 
 
-def is_list_alike(container):
+def is_list_alike(container: Union[str, Sequence]) -> bool:
     return isinstance(container, collections.abc.Sequence) and not isinstance(container, str)
 
 
@@ -168,7 +155,7 @@ def urlsafe_address(address):
     return addr, port
 
 
-def decode(data: bytes, encoding: str) -> bytes:
+def decode(data: bytes, encoding: str) -> Union[None, str, bytes]:
     """Attempt to decode data based on the supplied encoding.
 
     If decoding fails a ValueError is raised.
diff --git a/seleniumwire/webdriver.py b/seleniumwire/webdriver.py
index 77be642..b53972b 100644
--- a/seleniumwire/webdriver.py
+++ b/seleniumwire/webdriver.py
@@ -54,25 +54,25 @@ def proxy(self) -> Dict[str, Any]:
         """Get the proxy configuration for the driver."""
 
         conf = {}
-        mode = getattr(self.backend.master.options, 'mode')
+        mode = getattr(self.backend.master.options, 'mode')  # type: ignore
 
         if mode and mode.startswith('upstream'):
             upstream = mode.split('upstream:')[1]
             scheme, *rest = upstream.split('://')
 
-            auth = getattr(self.backend.master.options, 'upstream_auth')
+            auth = getattr(self.backend.master.options, 'upstream_auth')  # type: ignore
 
             if auth:
                 conf[scheme] = f'{scheme}://{auth}@{rest[0]}'
             else:
                 conf[scheme] = f'{scheme}://{rest[0]}'
 
-        no_proxy = getattr(self.backend.master.options, 'no_proxy')
+        no_proxy = getattr(self.backend.master.options, 'no_proxy')  # type: ignore
 
         if no_proxy:
             conf['no_proxy'] = ','.join(no_proxy)
 
-        custom_auth = getattr(self.backend.master.options, 'upstream_custom_auth')
+        custom_auth = getattr(self.backend.master.options, 'upstream_custom_auth')  # type: ignore
 
         if custom_auth:
             conf['custom_authorization'] = custom_auth
@@ -93,7 +93,9 @@ def proxy(self, proxy_conf: Dict[str, Any]):
         Args:
             proxy_conf: The proxy configuration.
         """
-        self.backend.master.options.update(**build_proxy_args(get_upstream_proxy({'proxy': proxy_conf})))
+        self.backend.master.options.update(  # type: ignore
+            **build_proxy_args(get_upstream_proxy({'proxy': proxy_conf}))
+        )
 
 
 class Firefox(InspectRequestsMixin, DriverCommonMixin, _Firefox):
diff --git a/tests/seleniumwire/test_utils.py b/tests/seleniumwire/test_utils.py
index 927466f..60aa6f1 100644
--- a/tests/seleniumwire/test_utils.py
+++ b/tests/seleniumwire/test_utils.py
@@ -241,9 +241,10 @@ def test_extract_cert(self, mock_pkgutil, mock_getcwd):
         m_open.assert_called_once_with(Path('cwd', 'ca.crt'), 'wb')
         m_open.return_value.write.assert_called_once_with(b'cert_data')
 
+    @patch('seleniumwire.utils.log')
     @patch('seleniumwire.utils.pkgutil')
-    def test_extract_cert_not_found(self, mock_pkgutil):
-        mock_pkgutil.get_data.side_effect = FileNotFoundError
+    def test_extract_cert_not_found(self, mock_pkgutil, mock_log):
+        mock_pkgutil.get_data.return_value = None
         m_open = mock_open()
 
         with patch('seleniumwire.utils.open', m_open):
@@ -251,6 +252,7 @@ def test_extract_cert_not_found(self, mock_pkgutil):
 
         mock_pkgutil.get_data.assert_called_once_with('seleniumwire', 'foo.crt')
         m_open.assert_not_called()
+        mock_log.error.assert_called_once()
 
     @patch('seleniumwire.utils.os')
     @patch('seleniumwire.utils.pkgutil')
@@ -291,6 +293,21 @@ def test_extract_cert_and_key_no_check(self, mock_path, mock_os):
 
         m_open.assert_called_once()
 
+    @patch('seleniumwire.utils.log')
+    @patch('seleniumwire.utils.os')
+    @patch('seleniumwire.utils.pkgutil')
+    @patch('seleniumwire.utils.Path')
+    def test_extract_cert_and_key_not_found(self, mock_path, mock_pkgutil, mock_os, mock_log):
+        mock_path.return_value.exists.return_value = False
+        mock_pkgutil.get_data.side_effect = (None, None)
+        m_open = mock_open()
+
+        with patch('seleniumwire.utils.open', m_open):
+            extract_cert_and_key(Path('some', 'path'))
+
+        m_open.assert_not_called()
+        mock_log.error.assert_called_once()
+
 
 def test_urlsafe_address_ipv4():
     assert urlsafe_address(('192.168.0.1', 9999)) == ('192.168.0.1', 9999)
diff --git a/tox.ini b/tox.ini
index 33cdef8..0e39c49 100644
--- a/tox.ini
+++ b/tox.ini
@@ -7,6 +7,7 @@ envlist =
     isort
     black
     flake8
+    mypy
 
 [testenv]
 setenv =
@@ -34,6 +35,12 @@ deps =
 commands =
     flake8 seleniumwire
 
+[testenv:mypy]
+deps =
+    mypy
+commands =
+    mypy seleniumwire tests
+
 [testenv:e2e]
 commands =
     pytest -s -vv --tb=native tests/end2end