Traditionally, compilers are black boxes -- source code goes in one end, and object files or assemblies come out the other end. The Roslyn project changes that model by opening up the VB and C# compilers. The compilers provide APIs to allow tools and end-users to share in the wealth of information the compilers have about code. The Microsoft "Roslyn" CTP previews the new language object models for code generation, analysis, and refactoring, and the upcoming support for scripting and interactive use of C# and Visual Basic.