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 MIS 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 water vapor 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). The MIS design will build on the capabilities of these heritage sensors to ensure data continuity for operations. Further improvements are planned for MIS in the areas of RFI mitigation techniques, sensor calibration, stability, channel selection, algorithm development and cal/val. As a result, increased utility of sea surface wind vector, sea surface temperature and soil moisture measurements as well as atmospheric sounding from MIS are anticipated. This paper will describe the nature of the new sensor design and expected EDR performance in these key areas.