diff --git a/docs/usage.rst b/docs/usage.rst
index 5e67ab9..bc6b846 100644
--- a/docs/usage.rst
+++ b/docs/usage.rst
@@ -1,7 +1,10 @@
-Usage (Or: How To Get The List of Libraries)
-============================================
+Usage
+=====
 
-The primary function that you'll care about in this package is ``stdlib_list.stdlib_list``.
+Getting The List of Libraries
+-----------------------------
+
+``stdlib_list.stdlib_list`` returns the list of libraries in stdlib for any given version (by default, current python version).
 
 In particular:
 
@@ -15,5 +18,30 @@ In particular:
     Out[3]: ['__future__', '__main__', '_dummy_thread', '_thread', 'abc', 'aifc']
 
 
+Checking if a Module is part of stdlib
+--------------------------------------
+
+``stdlib_list.in_stdlib`` provides an efficient way to check if a module name is part of stdlib.
+It relies on ``@lru_cache`` to cache the stdlib list and query results for similar calls. Therefore it is much more efficient than ``module_name in stdlib_list()`` especially if you wish to perform multiple checks.
+
+In particular:
+
+::
+
+    >>> from stdlib_list import in_stdlib
+    >>> in_stdlib('zipimport')  # built in
+    True
+    >>> in_stdlib('math')       # C-API stdlib module, but linked as extension (on my machine)
+    True
+    >>> in_stdlib('numpy')      # C-API extension, not stdlib
+    False
+    >>> in_stdlib('sys')        # built-in (and special)
+    True
+    >>> in_stdlib('os')         # Python code in stdlib
+    True
+    >>> in_stdlib('requests')   # Python code, not stdlib
+    False
+
+
 .. automodule:: stdlib_list
-    :members: stdlib_list
+    :members: stdlib_list, in_stdlib
diff --git a/setup.py b/setup.py
index fc6681c..f910d89 100644
--- a/setup.py
+++ b/setup.py
@@ -17,6 +17,7 @@
     author_email='jackmaney@gmail.com',
     url='https://github.com/jackmaney/python-stdlib-list',
     version=versioneer.get_version(),
+    install_requires=['functools32;python_version<"3.2"'],
     extras_require={"develop": ["sphinx"]},
     description='A list of Python Standard Libraries (2.6-7, 3.2-6).',
     long_description=long_description,
diff --git a/stdlib_list/__init__.py b/stdlib_list/__init__.py
index 549b317..d393949 100644
--- a/stdlib_list/__init__.py
+++ b/stdlib_list/__init__.py
@@ -3,4 +3,4 @@
 del get_versions
 
 # Import all the things that used to be in here for backwards-compatibility reasons
-from .base import stdlib_list, get_canonical_version, short_versions, long_versions, base_dir, list_dir
+from .base import stdlib_list, in_stdlib, get_canonical_version, short_versions, long_versions, base_dir, list_dir
diff --git a/stdlib_list/base.py b/stdlib_list/base.py
index 1a11d83..7b5c540 100644
--- a/stdlib_list/base.py
+++ b/stdlib_list/base.py
@@ -3,6 +3,11 @@
 import os
 import sys
 
+try:
+    from functools import lru_cache
+except ImportError:
+    from functools32 import lru_cache
+
 base_dir = os.path.dirname(os.path.realpath(__file__))
 
 list_dir = os.path.join(base_dir, "lists")
@@ -45,3 +50,36 @@ def stdlib_list(version=None):
         result = [y for y in [x.strip() for x in f.readlines()] if y]
 
     return result
+
+
+@lru_cache(maxsize=16)
+def _stdlib_list_with_cache(version=None):
+    """Internal cached version of `stdlib_list`"""
+    return stdlib_list(version=version)
+
+
+@lru_cache(maxsize=256)
+def in_stdlib(module_name, version=None):
+    """
+    Return a ``bool`` indicating if module ``module_name`` is in the list of stdlib
+    symbols for python version ``version``. If ``version`` is ``None`` (default), the
+    version of current python interpreter is used.
+
+    Note that ``True`` will be returned for built-in modules too, since this project
+    considers they are part of stdlib. See :issue:21.
+
+    It relies on ``@lru_cache`` to cache the stdlib list and query results for similar
+    calls. Therefore it is much more efficient than ``module_name in stdlib_list()``
+    especially if you wish to perform multiple checks.
+
+    :param str|None module_name: The module name (as a string) to query for.
+    :param str|None version: The version (as a string) whose list of libraries you want
+    (one of ``"2.6"``, ``"2.7"``, ``"3.2"``, ``"3.3"``, ``"3.4"``, or ``"3.5"``).
+    If not specified, the current version of Python will be used.
+
+    :return: A bool indicating if the given module name is part of standard libraries
+    for the specified version of Python.
+    :rtype: list
+    """
+    ref_list = _stdlib_list_with_cache(version=version)
+    return module_name in ref_list