Scatterometer instruments are active microwave sensors that transmit a series of microwave pulses and measure the retumed echo power to allow determination of the normalized radar backscattering cross section (sigma-0) of the ocean surface and, subsequently, the ocean wind speed and direction The NASA Scatterometer (NSCAT) was launched on board of the ADEOS in August 1996 and retumed 10 months of high quality data before the failure of the ADEOS spacecraft terminated the data stream in June, 1997. Since the next scheduled NASA scatterometer mission, SeaWindrs on ADEOS ZI, will be launched in mid-2000, NASA has approved a NSCAT replacement mission, called SeaWinds on QuikSCAT (QSCAT), to be launched in November 1998. The purpose of this paper is to provide a general description of the spacecraft system and the instrument design.