Microsoft Expression Blend is a user interface design tool developed and sold by Microsoft for creating graphical interfaces for web and desktop applications that blend the features of these two types of applications. Expression Blend 4 is the fourth version of Expression Blend released in May 2010, it adds support for Silverlight 4 and WPF 4 while also remaining compatible with Silverlight 3 and WPF 3.5 SP1 applications.