The Global Precipitation Measurement Mission is an international space network of satellites designed to provide the next generation precipitation observations around the world every 2 to 4 hours. It is a science mission with integrated applications goals. The GPM concept centers on the deployment of a Core Spacecraft carrying advanced active and passive microwave sensors to function as a physics observatory to gain physical insights into precipitating systems and to serve as a reference standard to improve global precipitation estimates from a constellation of research and operational microwave sensors capable of precipitation measurements. The GPM constellation buildup follows a progressive strategy to take advantage of partner satellites that carry microwave sensors such as a conical-scanning imager or a cross-track-scanning sounder.