This document introduces Xamarin, a platform for building native mobile apps for iOS and Android using C# and F#. It allows sharing of code across platforms while retaining native UIs, APIs, and performance. It discusses Xamarin's approach of building native apps with code sharing versus alternatives like building separately for each platform or using a lowest common denominator approach. It highlights Xamarin's language and library support, IDE integration, and tools like Xamarin.Forms for sharing UI code.