diff --git a/nox/_decorators.py b/nox/_decorators.py
index 153e331b..f94cef9a 100644
--- a/nox/_decorators.py
+++ b/nox/_decorators.py
@@ -18,7 +18,7 @@ def __new__(
         return cast("FunctionDecorator", functools.wraps(func)(obj))
 
 
-def _copy_func(src: Callable, name: str = None) -> Callable:
+def _copy_func(src: Callable, name: Optional[str] = None) -> Callable:
     dst = types.FunctionType(
         src.__code__,
         src.__globals__,  # type: ignore
@@ -41,7 +41,7 @@ def __init__(
         name: Optional[str] = None,
         venv_backend: Any = None,
         venv_params: Any = None,
-        should_warn: Dict[str, Any] = None,
+        should_warn: Optional[Dict[str, Any]] = None,
     ):
         self.func = func
         self.python = python
@@ -53,7 +53,7 @@ def __init__(
     def __call__(self, *args: Any, **kwargs: Any) -> Any:
         return self.func(*args, **kwargs)
 
-    def copy(self, name: str = None) -> "Func":
+    def copy(self, name: Optional[str] = None) -> "Func":
         return Func(
             _copy_func(self.func, name),
             self.python,
diff --git a/nox/_option_set.py b/nox/_option_set.py
index 4bcbb357..0da46fb0 100644
--- a/nox/_option_set.py
+++ b/nox/_option_set.py
@@ -20,7 +20,8 @@
 import argparse
 import collections
 import functools
-from argparse import ArgumentError, ArgumentParser, Namespace
+from argparse import ArgumentError as ArgumentError
+from argparse import ArgumentParser, Namespace
 from typing import Any, Callable, List, Optional, Tuple, Union
 
 import argcomplete
diff --git a/nox/_version.py b/nox/_version.py
index 740b7fd3..2e247534 100644
--- a/nox/_version.py
+++ b/nox/_version.py
@@ -36,7 +36,7 @@ class InvalidVersionSpecifier(Exception):
 
 def get_nox_version() -> str:
     """Return the version of the installed Nox package."""
-    return metadata.version("nox")
+    return metadata.version("nox")  # type: ignore
 
 
 def _parse_string_constant(node: ast.AST) -> Optional[str]:  # pragma: no cover
diff --git a/nox/command.py b/nox/command.py
index 8d915054..2c3f1095 100644
--- a/nox/command.py
+++ b/nox/command.py
@@ -21,11 +21,16 @@
 from nox.logger import logger
 from nox.popen import popen
 
+if sys.version_info < (3, 8):  # pragma: no cover
+    from typing_extensions import Literal
+else:  # pragma: no cover
+    from typing import Literal
+
 
 class CommandFailed(Exception):
     """Raised when an executed command returns a non-success status code."""
 
-    def __init__(self, reason: str = None) -> None:
+    def __init__(self, reason: Optional[str] = None) -> None:
         super(CommandFailed, self).__init__(reason)
         self.reason = reason
 
@@ -70,7 +75,7 @@ def run(
     paths: Optional[List[str]] = None,
     success_codes: Optional[Iterable[int]] = None,
     log: bool = True,
-    external: bool = False,
+    external: Union[Literal["error"], bool] = False,
     **popen_kws: Any
 ) -> Union[str, bool]:
     """Run a command-line program."""
diff --git a/nox/popen.py b/nox/popen.py
index 9bd1c115..010dd321 100644
--- a/nox/popen.py
+++ b/nox/popen.py
@@ -16,7 +16,7 @@
 import locale
 import subprocess
 import sys
-from typing import IO, Mapping, Sequence, Tuple, Union
+from typing import IO, Mapping, Optional, Sequence, Tuple, Union
 
 
 def shutdown_process(proc: subprocess.Popen) -> Tuple[bytes, bytes]:
@@ -54,9 +54,9 @@ def decode_output(output: bytes) -> str:
 
 def popen(
     args: Sequence[str],
-    env: Mapping[str, str] = None,
+    env: Optional[Mapping[str, str]] = None,
     silent: bool = False,
-    stdout: Union[int, IO] = None,
+    stdout: Optional[Union[int, IO]] = None,
     stderr: Union[int, IO] = subprocess.STDOUT,
 ) -> Tuple[int, str]:
     if silent and stdout is not None:
diff --git a/nox/sessions.py b/nox/sessions.py
index cabaae12..0cdbe78e 100644
--- a/nox/sessions.py
+++ b/nox/sessions.py
@@ -211,7 +211,7 @@ def _run_func(
             raise nox.command.CommandFailed()
 
     def run(
-        self, *args: str, env: Mapping[str, str] = None, **kwargs: Any
+        self, *args: str, env: Optional[Mapping[str, str]] = None, **kwargs: Any
     ) -> Optional[Any]:
         """Run a command.
 
@@ -269,7 +269,7 @@ def run(
         return self._run(*args, env=env, **kwargs)
 
     def run_always(
-        self, *args: str, env: Mapping[str, str] = None, **kwargs: Any
+        self, *args: str, env: Optional[Mapping[str, str]] = None, **kwargs: Any
     ) -> Optional[Any]:
         """Run a command **always**.
 
@@ -311,7 +311,9 @@ def run_always(
 
         return self._run(*args, env=env, **kwargs)
 
-    def _run(self, *args: str, env: Mapping[str, str] = None, **kwargs: Any) -> Any:
+    def _run(
+        self, *args: str, env: Optional[Mapping[str, str]] = None, **kwargs: Any
+    ) -> Any:
         """Like run(), except that it runs even if --install-only is provided."""
         # Legacy support - run a function given.
         if callable(args[0]):
diff --git a/nox/tasks.py b/nox/tasks.py
index c23b118d..60b5f9a6 100644
--- a/nox/tasks.py
+++ b/nox/tasks.py
@@ -63,7 +63,7 @@ def load_nox_module(global_config: Namespace) -> Union[types.ModuleType, int]:
         os.chdir(os.path.realpath(os.path.dirname(global_config.noxfile)))
         return importlib.machinery.SourceFileLoader(
             "user_nox_module", global_config.noxfile
-        ).load_module()  # type: ignore
+        ).load_module()
 
     except (VersionCheckFailed, InvalidVersionSpecifier) as error:
         logger.error(str(error))
diff --git a/nox/virtualenv.py b/nox/virtualenv.py
index 13f3fbc3..a6bf8448 100644
--- a/nox/virtualenv.py
+++ b/nox/virtualenv.py
@@ -53,7 +53,9 @@ class ProcessEnv:
     # Special programs that aren't included in the environment.
     allowed_globals = ()  # type: _typing.ClassVar[Tuple[Any, ...]]
 
-    def __init__(self, bin_paths: None = None, env: Mapping[str, str] = None) -> None:
+    def __init__(
+        self, bin_paths: None = None, env: Optional[Mapping[str, str]] = None
+    ) -> None:
         self._bin_paths = bin_paths
         self.env = os.environ.copy()
         self._reused = False
diff --git a/noxfile.py b/noxfile.py
index 9e247bb2..58967c2c 100644
--- a/noxfile.py
+++ b/noxfile.py
@@ -86,15 +86,16 @@ def blacken(session):
 
 @nox.session(python="3.8")
 def lint(session):
-    session.install("flake8==3.9.2", "black==21.5b2", "isort==5.8.0", "mypy==0.812")
-    session.run(
-        "mypy",
-        "--config-file=",
-        "--disallow-untyped-defs",
-        "--warn-unused-ignores",
-        "--ignore-missing-imports",
-        "nox",
+    session.install(
+        "flake8==3.9.2",
+        "black==21.6b0",
+        "isort==5.8.0",
+        "mypy==0.902",
+        "types-jinja2",
+        "packaging",
+        "importlib_metadata",
     )
+    session.run("mypy")
     files = ["nox", "tests", "noxfile.py", "setup.py"]
     session.run("black", "--check", *files)
     session.run("isort", "--check", *files)
diff --git a/pyproject.toml b/pyproject.toml
index f6c16894..90bcd11d 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -4,3 +4,26 @@ requires = [
     "wheel",
 ]
 build-backend = "setuptools.build_meta"
+
+
+[tool.mypy]
+files = ["nox"]
+python_version = "3.6"
+warn_unused_configs = true
+disallow_any_generics = false
+disallow_subclassing_any = false
+disallow_untyped_calls = true
+disallow_untyped_defs = true
+disallow_incomplete_defs = true
+check_untyped_defs = true
+disallow_untyped_decorators = true
+no_implicit_optional = true
+warn_redundant_casts = true
+warn_unused_ignores = true
+warn_return_any = false
+no_implicit_reexport = true
+strict_equality = true
+
+[[tool.mypy.overrides]]
+module = [ "argcomplete", "colorlog.*", "py", "tox.*" ]
+ignore_missing_imports = true
\ No newline at end of file
diff --git a/setup.py b/setup.py
index 28d99cd4..92e3d6a1 100644
--- a/setup.py
+++ b/setup.py
@@ -54,6 +54,7 @@
         "colorlog>=2.6.1,<7.0.0",
         "packaging>=20.9",
         "py>=1.4.0,<2.0.0",
+        "typing_extensions>=3.7.4; python_version < '3.8'",
         "virtualenv>=14.0.0",
         "importlib_metadata; python_version < '3.8'",
     ],