The Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) is a joint Taiwan-U.S. space mission, with a plan to launch a constellation of six micro-satellites in late 2005. Each satellite will carry three instruments: a Global Positioning System (GPS) Radio Occultation (RO) receiver, a Tiny Ionospheric Photometer (TIP), and a Tri-Band Beacon (TBB). The COSMIC constellation will provide up to 3,000 RO soundings that are distributed relatively uniformly around the Earth. The raw measurements made by the GPS RO receivers are the phase and amplitude of the GPS radio signals (L-band with wavelengths L1 ~19.0 cm and L2 ~ 24.4 cm), which can be used to derive the vertical profiles of temperature, moisture and electron density. The TIP and TBB instruments will provide additional ionospheric measurements. The COSMIC data from these three instruments are expected to make a significant impact on global weather prediction, climate and ionosphere monitoring and research. This paper presents (1) an overview of the COSMIC system; (2) CDAAC results from two recent GPS RO missions, CHAMP and SAC-C; and (3) the potential impact of COSMIC data on numerical weather prediction as indicated by recent observing system simulation experiments (OSSEs).