The GPM mission has four functional components (the core satellite, the constellation satellites, the precipitation processing system, and the international ground validation program) that are the result of an international, collaborative effort to expand capabilities for global, space-based precipitation measurements. GPM activities at NOAA and NASA will occur over the three phases of the mission: the pre-launch phase, the post-launch phase, and the potential NOAA-takeover phase. The GPM mission is a prototype for the international GEOSS initiative, which will require an integrated, intercalibration system for space-based instruments. The following chapters provide a framework for NOAA activities—both within the agency and with agency partners—during the three phases of the GPM mission.