The National Polar-orbiting Operational Environmental Satellite System (NPOESS) is developing a replacement for the Conical-scanning Microwave Imager/Sounder (CMIS), the Microwave Imager/Sounder (MIS). The Microwave Image/Sounder will perform key measurements for the NPOESS system including Soil Moisture and Sea Surface Winds. Other environmental parameters related to conical-scanning microwave radiometers and MIS include Atmospheric Temperature and Moisture Profiles, Sea Surface Temperature, integrated atmospheric moisture and precipitation measurements. The importance of MIS-related measurements to meteorology, weather forecasting and climatology is demonstrated by the role of measurements from several MIS-legacy sensors including the DMSP Special Sensor Microwave Imager (SSM/I) and Imager/Sounder (SSMIS), as well as the TRMM Microwave Imager (TMI), the EOS Advanced Microwave Scanning Radiometer (AMSR-E), and the Satellite-based Vector Winds Measuring System (WindSat). These sensors provide important insight regarding the design, operation and utility of the future NPOESS MIS in areas of RFI mitigation techniques, sensor calibration and stability, and channel selection; algorithm development and cal/val considerations; the impact of SSMIS measurements for improving NWP forecasts, and improvements to sea surface wind vector, sea surface temperature and soil moisture measuring capabilities. Drawing heavily on these experiences and the diverse background they supply, this paper will describe the role of MIS performance capabilities within NPOESS and relate them to the current MIS system definition activities.