The Constellation Observing System for Meteorology Ionosphere and Climate (COSMIC) is a satellite mission for the Earth sciences that will make use of recent developments in remote sensing, communications tech nology, and computing to solve some of the most important geo-scientific issues today. COSMIC plans to launch eight Low Earth Orbit (LEO) satel lites in 2003. Each of these spacecraft will carry three science payloads for weather and space weather research and prediction, climate monitoring, and geodesy: 1) GPS occultation receiver, 2) Tiny Ionospheric Photometer (TIP), and 3) Triband Beacon transmitters (TBB). Each of the LEOs will track the GPS satellites as they are occulted behind the Earth limb to re trieve up to 500 daily profiles of key ionospheric and atmospheric proper ties. Every day the constellation will provide globally about 4000 GPS sound ings. TIP will measure electron densities at the peak of the F2 layer along the satellite track. TBB transmissions will be received on the ground for high-resolution tomographic reconstruction of the ionospheric electron density. COSMIC continuos precise tracking of all GPS satellites in view, also promise to benefit geodetic studies. The COSMIC system includes the LEO satellites, ground data reception and spacecraft control stations, data analysis centers and the data communications networks. This paper gives a COSMIC science overview and describes the COSMIC system. (