This document discusses building cross-platform applications using Portable Class Libraries. Portable Class Libraries allow developers to share common code across multiple platforms like Windows, Windows Phone, iOS and Android. It addresses the challenges of building native apps for different platforms and when Portable Class Libraries are appropriate. It provides an overview of what APIs are available in Portable Class Libraries and how to structure applications using the Model-View-ViewModel pattern to maximize shared and platform-specific code. Resources for learning more about Portable Class Libraries and cross-platform development are also included.