The Airborne Earth Science Microwave Imaging Radiometer (AESMIR) is a passive microwave airborne imager covering the 6-100 GHz bands that are essential for observing key Earth System elements such as precipitation, snow, soil moisture, ocean winds, sea ice, sea surface temperature, vegetation, etc. AESMIR’s channels are configured to enable it to simulate various channels on multiple satellite radiometers, including AMSR-E, SSMI, SSMIS, AMSU, ATMS, TMI, GMI, ATMS, & MIS. Programmable scan modes include conical and cross-track scanning. As such, AESMIR can serve as an inter-satellite calibration tool for constellation missions (e.g., GPM) as well as for long-term multi-satellite data series (Climate Data Records). The most unique/cutting edge feature of the instrument is its coverage of key water cycle microwave bands in a single mechanical package—making efficient & cost-effective use of limited space on research aircraft, and maximizing the possibilities for co-flying with other instruments to provide synergistic science. State-of-the-art calibration, fully-polarimetric (4-Stokes) observations, and the ability to accommodate large/heavy sensors (up to 300 kg) are other features of AESMIR. AESMIR currently flies on the NASA P-3 aircraft. With these capabilities, AESMIR is an Earth Science facility for new microwave remote sensing discovery, pre-launch algorithm development, and post-launch Calibration/Validation activities, as well as serving as a technology risk reduction testbed for upcoming spaceborne radiometers. In the latter role, AESMIR is already supporting the GPM, Aquarius, and SMAP missions.