Portable Class Libraries were introduced with Visual Studio 2010 SP1 to aid writing libraries that could be used on many different platforms - the full .NET 4/4.5 framework, Windows Phone, Silverlight, Xbox, and Windows Store apps. You simply select which platforms and versions you want to target, then the available subset of APIs are magically available. But how does it work? How does Visual Studio know what it can target, and how does the same assembly run on many different platforms? Today, I'll be finding out.