Passive microwave sounders operating at millimeter wavelengths, in concert with other sensors and improved assimilation and modeling techniques, have extended the nowcast and forecast capabilities of NOAA and DoD operational systems for severe weather. Further improvements will result from the NPOESS assets to be launched later this decade. These contributions began with the operational MSU and SSM/T microwave sounders in the late 1970’s, which applied technologies proven in the NASA NIMBUS-5/6/7 programs, and have continued with improved sensors such as AMSU-A/B, SM/T2, and others. The planned NPOESS ATMS and CMIS microwave imaging sounders will provide further advances. For example, ATMS will have three significant advantages over its predecessors: 1) wide-swath scans that leave no voids at the equator between successive orbits and that overlap more at higher latitudes, 2) ~33-km resolution versus ~50 km for sounding temperature, precipitation, and hurricane cores, and 3) denser sampling cross-track so as to reduce aliasing. In addition the conically scanning CMIS sensor will provide additional channels with higher spatial resolution for monitoring surface phenomena, clouds, precipitation, and several other parameters to improve our understanding of severe weather and its prediction.