For Microsoft .NET is the platform of choice for software development. In this talk we demonstrate how to design and implement large applications with .NET technology. Based on the experience of several .NET projects we explain design concepts and architecture for client-server systems, web applications and web services that were successfully implemented in projects. Useful extensions of the development environment Visual Studio.NET for team development are explained. Finally we show typical problems during the development and their solution as well as proven concepts for the design of standard components such as GUI, communication, data access and integration of existing applications.