This document discusses using the Xamarin platform to build Android apps using C#. Key points include: - Xamarin allows developing Android apps using C# and sharing code with other platforms like iOS and Windows. - It works by using Mono and bindings to expose the Android platform in C# and allow calling Java libraries. The compiler produces IL that runs natively on Android. - Developers can use Xamarin Studio or Visual Studio and the apps have native performance since they run natively on Android.