About X11 and OS X Mountain Lion
Read about X11 and Mountain Lion.
X11 is not included with Mountain Lion, but X11 server and client libraries for OS X Mountain Lion are available from the XQuartz project: http://xquartz.macosforge.org. You should use XQuartz version 2.7.2 or later.
Apple created the XQuartz project as a community effort to further develop and support X11 on OS X. The XQuartz project was originally based on the version of X11 included in Mac OS X v10.5, and there have been multiple releases with fixes, support for new OS X features, and additional refinements to the X11 experience. Many of these enhancements were included in X11 for Mac OS X v10.6 and OS X Lion. Apple is a contributor to the XQuartz project and has worked to ensure that X11 works as expected in OS X Mountain Lion with XQuartz 2.7.2 or later.