The Global Precipitation Climatology Project (GPCP) is an element of the Global Energy and Water Cycle Experiment (GEWEX) of the World Climate Research program (WCRP). It was established by the WCRP in 1986 with the initial goal of providing monthly mean precipitation data on a 2.5°— 2.5° latitude -longitude grid. Monthly mean precipitation estimates are being produced beginning in 1979 and planned to go through 2005. The GPCP has accomplished this by merging infrared and microwave satellite estimates of precipitation with rain gauge data from more than 6,000 stations. Infrared precipitation estimates are obtained from GOES (United States), GMS (Japan) and Meteosat (European Community) geostationary satellites and National Oceanic and Atmospheric Administration (NOAA) operational polar orbiting satellites. Microwave estimates are obtained from the U.S. Defense Meteorological Satellite Program (DMSP) satellites using the Special Sensor Microwave Imager (SSM/I). These data sets will be used to validate general circulation and climate models, study the global hydrological cycle and diagnose the variability of the global climate system. Data sets have been expanded so that in addition to the monthly mean product available, the GPCP now has a 2.5°—2.5° degree pentad data set starting in 1979 and a 1°—1° daily data set starting in 1997.