Properly detailed and built, the rain screen wall is an effective envelope solution to external moisture penetration. This article reviews the various forces that move rain into buildings and suggests how they can be counteracted. Aspects of building geometry and aerodynamics are discussed as well as the components of wall assemblies. The rain screen principle and the process of pressure equalization are explained. Parameters are given for determining how to vent a rain screen properly. Sample details are shown for various rain screen wall and joint systems. Ten key features to look for in rain screen details are provided.