Since 1979, Remote Sensing Systems has been providing a global community of researchers and decision makers with inter-calibrated microwave measurements and geophysical retrievals derived from passive and active spaceborne sensors. These datasets, from 35 microwave sensors covering a time period of 40 years, have been consolidated at the Microwave Climate Data Center repository. The geophysical retrievals include: sea-surface temperature, near-surface ocean wind speed and direction, columnar atmospheric water vapor, columnar cloud liquid water, sea-surface rain rate, seasurface salinity, and atmospheric temperature profiles. Consistent calibration procedures and retrieval methods have been applied during the data processing to ensure these datasets are suitable for climate research. All of the geophysical retrievals relate to the air-sea boundary layer and are classified as essential climate variables by the Global Climate Observing System. In this paper, we give an overview of the microwave sensors, the inter-calibration methods, the retrieval algorithms, and the air-sea essential climate variable datasets housed at the Microwave Climate Data Center.