This document introduces MacRuby, a Ruby implementation that allows Ruby code to be used to script and build applications for Mac OS X. MacRuby provides a complete Ruby 1.9 environment and allows Ruby code to interface with Cocoa and Objective-C to build graphical user interface applications. Performance benchmarks show that MacRuby code runs nearly as fast as native Objective-C. MacRuby is currently available as a preview release but will be included in Mac OS X 10.7 Lion.