diff --git a/docs/source/configure.rst b/docs/source/configure.rst
index df5cbe2e..c340f5ee 100644
--- a/docs/source/configure.rst
+++ b/docs/source/configure.rst
@@ -4,23 +4,36 @@
 Configuration Overview
 ======================
 
-Gunicorn pulls configuration information from three distinct places.
+Gunicorn reads configuration information from five places.
 
-The first place that Gunicorn will read configuration from is the framework
-specific configuration file. Currently this only affects Paster applications.
+Gunicorn first reads environment variables for some configuration
+:ref:`settings <settings>`.
 
-The second source of configuration information is a configuration file that is
-optionally specified on the command line. Anything specified in the Gunicorn
-config file will override any framework specific settings.
+Gunicorn then reads configuration from a framework specific configuration
+file. Currently this only affects Paster applications.
+
+The third source of configuration information is an optional configuration file
+``gunicorn.conf.py`` searched in the current working directory or specified
+using a command line argument. Anything specified in this configuration file
+will override any framework specific settings.
+
+The fourth place of configuration information are command line arguments
+stored in an environment variable named ``GUNICORN_CMD_ARGS``.
 
 Lastly, the command line arguments used to invoke Gunicorn are the final place
 considered for configuration settings. If an option is specified on the command
 line, this is the value that will be used.
 
+When a configuration file is specified in the command line arguments and in the
+``GUNICORN_CMD_ARGS`` environment variable, only the configuration
+file specified on the command line is used.
+
 Once again, in order of least to most authoritative:
-    1. Framework Settings
-    2. Configuration File
-    3. Command Line
+    1. Environment Variables
+    2. Framework Settings
+    3. Configuration File
+    4. ``GUNICORN_CMD_ARGS``
+    5. Command Line
 
 
 .. note::
diff --git a/docs/source/settings.rst b/docs/source/settings.rst
index e293307b..01d49883 100644
--- a/docs/source/settings.rst
+++ b/docs/source/settings.rst
@@ -1165,6 +1165,10 @@ Multiple addresses can be bound. ex.::
 will bind the `test:app` application on localhost both on ipv6
 and ipv4 interfaces.
 
+If the ``PORT`` environment variable is defined, the default
+is ``['0.0.0.0:$PORT']``. If it is not defined, the default
+is ``['127.0.0.1:8000']``.
+
 .. _backlog:
 
 backlog
diff --git a/gunicorn/config.py b/gunicorn/config.py
index f220aa32..1032dae6 100644
--- a/gunicorn/config.py
+++ b/gunicorn/config.py
@@ -579,6 +579,10 @@ class Bind(Setting):
 
         will bind the `test:app` application on localhost both on ipv6
         and ipv4 interfaces.
+
+        If the ``PORT`` environment variable is defined, the default
+        is ``['0.0.0.0:$PORT']``. If it is not defined, the default
+        is ``['127.0.0.1:8000']``.
         """