MacRuby is an implementation of the Ruby language that runs on the Objective-C runtime under OS X. MacRuby is based on Ruby 1.9 but contains substantial modifications including the merging of object models, using the Objective-C 2.0 generational garbage collector, moving core types atop their Objective-C counterparts and replacement of standard libraries to more optimally integrate with OS X. MacRuby also includes a new library, HotCocoa, a thin, idiomatic Ruby layer that sits above Cocoa and other frameworks. This talk introduces MacRuby and HotCocoa and demonstrates how to use them to quickly build OS X desktop applications with Ruby.