Abstract: The Global Position System (GPS) radio occultation technique has emerged as a powerful and relatively inexpensive approach to sounding the global atmosphere in all weathers. As demonstrated by the proof of concept GPS meteorology (GPS/MET) experiment and more recently by the CHAMP and SAC-C missions, the GPS radio occultation (RO) sounding data are shown to be of high precision, accuracy and vertical resolution. On 15 April 2006, as the joint U.S.Taiwan COSMIC/FORMOSAT-3 (hereafter COSMIC) mission, a constellation of six microsatellites was successfully launched, marking the beginning of a new era of GPS atmospheric remote sensing. Under this background, we review the objectives and status of the GPS satellite missions and present highlights of the GPS RO data applications to weather research. Especially we describe in detail the operational assimilation of COSMIC data in the numerical weather prediction models.