After successful launches of a generation of the Advanced Microwave Sounding Unit-A (AMSU-A), AMSU-B, and the Microwave Humidity Souder (MHS) onboard NOAA-15 to NOAA-19 and METOP-A, the next generation of microwave sounders will be the Advanced Technology Microwave Sounder (ATMS) which will be flown on the National Polar-orbiting Operational Environmental Satellite System (NPOESS) starting with the NPOESS Preparatory Program (NPP) in October 2011. The design of ATMS follows closely to the currently operational microwave sensor suite of AMSU-A, AMSU-B, and MHS flown on the NOAA Polar-orbiting Environmental Satellites (POES) and METOP-A. The basic functionality of ATMS is similar to that of AMSU-A and AMSU–B (or MHS). But there are also some significant changes in the ATMS construction and operation, particularly with the increased number of Earth field of views (FOV) and faster rate of scanning. ATMS has two antenna systems of parabolic reflectors, one of which provides 15 channels (Ch. 1-15) in the range of 23.8 to 57.29 GHz for temperature sounding, and the other one contains channels 16 -22 for humidity sounding. All of these radiometers were tested and calibrated by the instrument contractors before launches. These pre-launch calibration data are analyzed to derive the calibration parameters input data set (CPIDS) used in the operational calibration algorithm to generate the NOAA Level 1B data sets for public distribution to general data users. A systematic post-launch calibration and validation of the instrumental performances were conducted with on-orbit data. Results from both pre- and post-launch calibrations and long-term monitoring of the instrument performances will be presented and discussed. Determination of Intersatellite/intersensor Calibration Biases (ICBs) over Antarctica will also be demonstrated.