diff --git a/.travis.yml b/.travis.yml
index 80c38c6..692b3c5 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -12,6 +12,6 @@ env:
     - TOXENV=pypy3
     - TOXENV=flake8
 install:
-  - travis_retry pip install tox
+    - travis_retry make install
 script:
-  - make test
+    - make test
diff --git a/Makefile b/Makefile
index c0e4ef4..570d4bf 100644
--- a/Makefile
+++ b/Makefile
@@ -8,6 +8,9 @@ help:
 	@echo '   make test         run the test suite'
 	@echo ''
 
+install:
+	pip install -e .[testing]
+
 release:
 	python setup.py register sdist bdist_wheel upload
 
diff --git a/README.rst b/README.rst
index e154392..d0ec20d 100644
--- a/README.rst
+++ b/README.rst
@@ -182,9 +182,19 @@ Read from a .env file (line delimited KEY=VALUE):
     # Values can be read as normal
     env.int('FOO')
 
+Contributing
+------------
+
+Clone the repo, create a virtualenv then run:
+
+    $ make install
+
+which will install the package as well any dependencies required for running the
+tests.
 
 Tests
------
+~~~~~
+
 .. image:: https://secure.travis-ci.org/rconradharris/envparse.png?branch=master
 
 To run the tests install tox::
diff --git a/envparse.py b/envparse.py
index e13b584..048edba 100644
--- a/envparse.py
+++ b/envparse.py
@@ -164,10 +164,13 @@ def cast(cls, value, cast=str, subcast=None):
     url = shortcut(urlparse.urlparse)
 
     @staticmethod
-    def read_envfile(path=None, **overrides):
+    def read_envfile(path=None, _overwrite=False, **overrides):
         """
         Read a .env file (line delimited KEY=VALUE) into os.environ.
 
+        If _overwrite is True, then existing environment variables will be overwritten by the
+        values within the file.
+
         If not given a path to the file, recurses up the directory tree until
         found.
 
@@ -179,6 +182,12 @@ def read_envfile(path=None, **overrides):
             caller_dir = os.path.dirname(frame.f_code.co_filename)
             path = os.path.join(os.path.abspath(caller_dir), '.env')
 
+        # Determine function to use for updating environment variables
+        if _overwrite:
+            set_env_var = os.environ.__setitem__
+        else:
+            set_env_var = os.environ.setdefault
+
         try:
             with open(path, 'r') as f:
                 content = f.read()
@@ -208,10 +217,11 @@ def read_envfile(path=None, **overrides):
             if not re.match(r'[A-Za-z_][A-Za-z_0-9]*', name):
                 continue
             value = value.replace(r'\n', '\n').replace(r'\t', '\t')
-            os.environ.setdefault(name, value)
+            set_env_var(name, value)
 
         for name, value in overrides.items():
-            os.environ.setdefault(name, value)
+            set_env_var(name, value)
+
 
 # Convenience object if no schema is required.
 env = Env()
diff --git a/setup.cfg b/setup.cfg
index 5e40900..d6e10b1 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -1,2 +1,5 @@
 [wheel]
 universal = 1
+
+[flake8]
+max-line-length = 100
diff --git a/setup.py b/setup.py
index 898814c..22956fd 100644
--- a/setup.py
+++ b/setup.py
@@ -1,6 +1,6 @@
 # -*- coding: utf-8 -*-
 from __future__ import print_function
-from setuptools import setup, find_packages
+from setuptools import setup
 from setuptools.command.test import test as TestCommand
 import codecs
 import os
@@ -36,11 +36,12 @@ def finalize_options(self):
         self.test_suite = True
 
     def run_tests(self):
-        #import here, cause outside the eggs aren't loaded
+        # import here, cause outside the eggs aren't loaded
         import pytest
         errno = pytest.main(self.pytest_args)
         sys.exit(errno)
 
+
 setup(
     name='envparse',
     version=find_version('.', 'envparse.py'),
@@ -56,7 +57,7 @@ def run_tests(self):
     py_modules=['envparse'],
     platforms='any',
     zip_safe=False,
-    classifiers = [
+    classifiers=[
         'Programming Language :: Python',
         'Programming Language :: Python :: 2',
         'Programming Language :: Python :: 3',
@@ -70,6 +71,6 @@ def run_tests(self):
         'Topic :: Internet :: WWW/HTTP :: Dynamic Content',
         ],
     extras_require={
-        'testing': ['pytest'],
+        'testing': ['pytest', 'tox', 'flake8', 'coverage'],
       }
 )
diff --git a/tests/test_read_envfile.py b/tests/test_read_envfile.py
new file mode 100644
index 0000000..a910760
--- /dev/null
+++ b/tests/test_read_envfile.py
@@ -0,0 +1,16 @@
+import os
+
+from envparse import env
+
+
+def test_existing_env_vars_can_be_overwritten():
+    os.environ['STR'] = 'bar'
+
+    # The value of STR is 'foo' in the sample file. Without the overwrite param, the env var should
+    # not be altered...
+    env.read_envfile("tests/envfile")
+    assert env.str("STR") == 'bar'
+
+    # ...but with the overwrite param it should.
+    env.read_envfile("tests/envfile", _overwrite=True)
+    assert env.str("STR") == 'foo'
diff --git a/tox.ini b/tox.ini
index 2405dc3..dba3b64 100644
--- a/tox.ini
+++ b/tox.ini
@@ -19,4 +19,4 @@ commands =
 deps =
     flake8
 commands =
-    flake8 envparse.py tests --max-line-length=100
+    flake8