On 15 April 2006, the joint Taiwan-U.S. COSMIC/FORMOSAT-3 (Constellation Observing System for Meteorology, Ionosphere, and Climate and Formosa Satellite mission #3; hereafter COSMIC) mission, a constellation of six microsatellites, was launched from Vandenberg Air Force Base in California. This marks the beginning of a new era for GPS (Global Positioning System) atmospheric remote sensing. The radio occultation (RO) technique, which makes use of radio signals transmitted by the Global Position System (GPS) satellites, has emerged as a powerful and relatively inexpensive approach for sounding the global atmosphere in all weather and over both land and ocean. The GPS RO soundings are shown to be of high precision, high accuracy, and high vertical resolution. Since its launch, COSMIC has been providing a large number of GPS RO soundings to support research and operational community. As of August 2008, COSMIC has taken 1.2 million neutral atmospheric RO soundings, and 1.5 million of ionospheric RO soundings, serving a total of 830 users from 46 countries. Currently, COSMIC is producing approximately 2,500 GPS RO soundings per day, uniformly distributed around the globe. Several global operational centers, including ECMWF, NCEP, UKMO, Meteo France, and MSC, are assimilating COSMIC data for global weather prediction, and have all demonstrated significant positive impacts. In this paper, we will present an overview and status of the COSMIC mission. We will highlight the significant contributions of COSMIC in weather prediction, climate monitoring, and ionospheric research. Conceptual design for a follow-on GPS RO constellation mission will also be discussed.