diff --git a/.mypy.ini b/.mypy.ini
new file mode 100644
index 0000000..be0671c
--- /dev/null
+++ b/.mypy.ini
@@ -0,0 +1,2 @@
+[mypy]
+strict = true
diff --git a/Makefile b/Makefile
index 7156972..ad66ea3 100644
--- a/Makefile
+++ b/Makefile
@@ -10,14 +10,14 @@ test_install:
 
 .PHONY: lint
 lint:
-	python -m mypy --strict src/pyprojroot
+	python -m mypy src/pyprojroot
 	python -m flake8 src/pyprojroot tests
 	python -m black --check --diff src/pyprojroot tests
 
 .PHONY: fmt
 fmt:
-	python -m black pyprojroot tests
+	python -m black src/pyprojroot tests
 
 .PHONY: test
 test:
-	python -m pytest
+	PYTHONPATH="src" python -m pytest
diff --git a/src/pyprojroot/__init__.py b/src/pyprojroot/__init__.py
index b17b019..a24c270 100644
--- a/src/pyprojroot/__init__.py
+++ b/src/pyprojroot/__init__.py
@@ -1,3 +1,11 @@
-from .criterion import *
-from .root import find_root, find_root_with_reason
-from .here import here
+from .criterion import (  # noqa: F401
+    CriterionFunction,
+    Criterion,
+    Criteria,
+    as_root_criterion,
+    has_file,
+    has_dir,
+    matches_glob,
+)
+from .root import find_root, find_root_with_reason  # noqa: F401
+from .here import here  # noqa: F401
diff --git a/src/pyprojroot/criterion.py b/src/pyprojroot/criterion.py
index a90094d..55408b4 100644
--- a/src/pyprojroot/criterion.py
+++ b/src/pyprojroot/criterion.py
@@ -5,9 +5,15 @@
 It is intended for interactive or programmatic only.
 """
 
-import pathlib as _pathlib
-import typing
-from os import PathLike as _PathLike
+__all__ = [
+    "CriterionFunction",
+    "Criterion",
+    "Criteria",
+    "as_root_criterion",
+    "has_file",
+    "has_dir",
+    "matches_glob",
+]
 
 # TODO: It would be nice to have a class that encapsulates these checks,
 # so that we can implement methods like |, !, &, ^ operators
@@ -15,18 +21,39 @@
 # TODO: Refactor in a way that allows creation of reasons
 
 
-def as_root_criterion(criterion) -> typing.Callable:
-    if callable(criterion):
+from pathlib import Path
+from typing import Union, Iterable, AnyStr
+from typing_extensions import Protocol, runtime_checkable
+
+from os import PathLike
+
+
+@runtime_checkable
+class CriterionFunction(Protocol):
+    def __call__(self, path: Path) -> bool:
+        ...
+
+
+Criterion = Union[CriterionFunction, "PathLike[AnyStr]"]
+
+
+Criteria = Iterable[Criterion]
+
+
+def as_root_criterion(criterion: Union[Criterion, Criteria]) -> CriterionFunction:
+    if isinstance(criterion, CriterionFunction):
         return criterion
 
     # criterion must be a Collection, rather than just Iterable
-    if isinstance(criterion, _PathLike):
-        criterion = [criterion]
-    criterion = list(criterion)
-
-    def f(path: _pathlib.Path) -> bool:
-        for c in criterion:
-            if isinstance(c, _PathLike):
+    criteria: Criteria
+    if isinstance(criterion, PathLike):
+        criteria = [criterion]
+    else:
+        criteria = list(criterion)
+
+    def f(path: Path) -> bool:
+        for c in criteria:
+            if isinstance(c, PathLike):
                 if (path / c).exists():
                     return True
             else:
@@ -37,38 +64,38 @@ def f(path: _pathlib.Path) -> bool:
     return f
 
 
-def has_file(file: _PathLike) -> typing.Callable:
+def has_file(file: Union[str, "PathLike[str]"]) -> CriterionFunction:
     """
     Check that specified file exists in path.
 
     Note that a directory with that name will not match.
     """
 
-    def f(path: _pathlib.Path) -> bool:
+    def f(path: Path) -> bool:
         return (path / file).is_file()
 
     return f
 
 
-def has_dir(file: _PathLike) -> typing.Callable:
+def has_dir(file: Union[str, "PathLike[str]"]) -> CriterionFunction:
     """
     Check that specified directory exists.
 
     Note that a regular file with that name will not match.
     """
 
-    def f(path: _pathlib.Path) -> bool:
+    def f(path: Path) -> bool:
         return (path / file).is_dir()
 
     return f
 
 
-def matches_glob(pat: str) -> typing.Callable:
+def matches_glob(pat: str) -> CriterionFunction:
     """
     Check that glob has at least one match.
     """
 
-    def f(path: _pathlib.Path) -> bool:
+    def f(path: Path) -> bool:
         matches = path.glob(pat)
         try:
             # Only need to get one item from generator
diff --git a/src/pyprojroot/here.py b/src/pyprojroot/here.py
index 4578aac..80a66a6 100644
--- a/src/pyprojroot/here.py
+++ b/src/pyprojroot/here.py
@@ -5,12 +5,15 @@
 It is intended for interactive use only.
 """
 
-import pathlib as _pathlib
-import warnings as _warnings
-from os import PathLike as _PathLike
+__all__ = ["CRITERIA", "get_here", "here"]
+
+from os import PathLike
+from pathlib import Path
+from typing import Union
+from warnings import warn
 
 from . import criterion
-from .root import find_root, find_root_with_reason
+from .root import find_root_with_reason
 
 CRITERIA = [
     criterion.has_file(".here"),
@@ -28,7 +31,7 @@
 
 def get_here():
     # TODO: This should only find_root once per session
-    start = _pathlib.Path.cwd()
+    start = Path.cwd()
     path, reason = find_root_with_reason(CRITERIA, start=start)
     return path, reason
 
@@ -36,7 +39,9 @@ def get_here():
 # TODO: Implement set_here
 
 
-def here(relative_project_path: _PathLike = "", warn_missing=False) -> _pathlib.Path:
+def here(
+    relative_project_path: Union[str, "PathLike[str]"] = "", warn_missing=False
+) -> Path:
     """
     Returns the path relative to the projects root directory.
     :param relative_project_path: relative path from project root
@@ -51,5 +56,5 @@ def here(relative_project_path: _PathLike = "", warn_missing=False) -> _pathlib.
         path = path / relative_project_path
 
     if warn_missing and not path.exists():
-        _warnings.warn(f"Path doesn't exist: {path!s}")
+        warn(f"Path doesn't exist: {path!s}")
     return path
diff --git a/src/pyprojroot/root.py b/src/pyprojroot/root.py
index b8ef64a..0354475 100644
--- a/src/pyprojroot/root.py
+++ b/src/pyprojroot/root.py
@@ -4,26 +4,27 @@
 
 It is intended for interactive or programmatic only.
 """
+__all__ = ["as_start_path", "find_root_with_reason", "find_root"]
 
-import pathlib as _pathlib
-import typing as _typing
-from os import PathLike as _PathLike
+from os import PathLike
+from pathlib import Path
+from typing import Tuple, Optional, Union
 
-from .criterion import as_root_criterion as _as_root_criterion
+from .criterion import as_root_criterion, Criterion, CriterionFunction
 
 
-def as_start_path(start: _PathLike) -> _pathlib.Path:
+def as_start_path(start: Optional[Union["PathLike[str]"]]) -> Path:
     if start is None:
-        return _pathlib.Path.cwd()
-    if not isinstance(start, _pathlib.Path):
-        start = _pathlib.Path(start)
+        return Path.cwd()
+    if not isinstance(start, Path):
+        start = Path(start)
     # TODO: consider `start = start.resolve()`
     return start
 
 
 def find_root_with_reason(
-    criterion, start: _PathLike = None
-) -> _typing.Tuple[_pathlib.Path, str]:
+    criterion: Criterion, start: Optional["PathLike[str]"] = None
+) -> Tuple[Path, str]:
     """
     Find directory matching root criterion with reason.
 
@@ -33,25 +34,25 @@ def find_root_with_reason(
     # TODO: Implement reasons
 
     # Prepare inputs
-    criterion = _as_root_criterion(criterion)
-    start = as_start_path(start)
+    root_criterion: CriterionFunction = as_root_criterion(criterion)
+    start_path: Path = as_start_path(start)
 
     # Check start
-    if start.is_dir() and criterion(start):
-        return start, "Pass"
+    if start_path.is_dir() and root_criterion(start_path):
+        return start_path, "Pass"
 
     # Iterate over all parents
     # TODO: Consider adding maximum depth
     # TODO: Consider limiting depth to path (e.g. "if p == stop: raise")
-    for p in start.parents:
-        if criterion(p):
+    for p in start_path.parents:
+        if root_criterion(p):
             return p, "Pass"
 
     # Not found
     raise RuntimeError("Project root not found.")
 
 
-def find_root(criterion, start: _PathLike = None, **kwargs) -> _pathlib.Path:
+def find_root(criterion: Criterion, start: Optional["PathLike[str]"] = None) -> Path:
     """
     Find directory matching root criterion.
 
@@ -59,7 +60,7 @@ def find_root(criterion, start: _PathLike = None, **kwargs) -> _pathlib.Path:
     matching root criterion.
     """
     try:
-        root, _ = find_root_with_reason(criterion, start=start, **kwargs)
+        root, _ = find_root_with_reason(criterion, start)
     except RuntimeError as ex:
         raise ex
     else: