The Global Precipitation Measurement (GPM) Microwave Imager (GMI) instrument is a multi-channel, conical- scanning, microwave radiometer serving an essential role in the near-global-coverage and frequent-revisit-time requirements of GPM. The instrumentation enables the Core spacecraft to serve as both a precipitation standard and as a radiometric standard for the other GPM constellation members. The GMI is characterized by thirteen microwave channels ranging in frequency from 10 GHz to 183 GHz. In addition to carrying channels similar to those on the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI), the GMI carries four high frequency, millimeter-wave, channels near 166 GHz and 183 GHz. With a 1.2 m diameter antenna, the GMI provides significantly improved spatial resolution over TMI.