We are pleased to announce that we just released Prism for Windows 8.1. Prism helps developers who want to create a Windows Store business app using C#, XAML, the Windows Runtime, and development patterns such as Model-View-ViewModel (MVVM) and event aggregation. Prism includes two libraries, a reference implementation called AdventureWorks Shopper, Quickstarts and associated documentation. This is an update from the version released in May for Windows 8. The guidance demonstrates: How to implement pages, touch, navigation, settings, suspend/resume, search, tiles, and tile notifications. How to implement the Model-View-ViewModel (MVVM) pattern. How to validate user input for correctness. How to manage application data. How to test your app and tune its performance.