Pre-flight and in-flight radiometric calibration plans are described for a multispectral optical imager of high spatial resolution for remote sensing of land surfaces and clouds from orbit, which will be launched in 1998 on NASA's EOS-AM1 spacecraft. The instrument named ASTER consists of three radiometers for three separate spectral regions, the visible and near-infrared radiometer (VNIR), the shortwave infrared radiometer (SWIR), and the thermal infrared radiometer (TIR). Absolute radiometric accuracy to better than 4% is required for the VNIR and SWIR radiance measurements, and from 1 K to 3 K, depending on the temperature region, from 200 K to 370 K for the TIR temperature measurements. The basic approach to in-flight calibration is to introduce a reference beam at the front end of the radiometer to calibrate the whole system in orbit. Thus the ASTER instrument is installed with internal on-board calibration units which include, as reference sources, incandescent lamps for the VNIR and SWIR and a black-body radiator for the TIR. Calibration reliability of the VNIR and SWIR is enhanced by a dual system of on-board calibration units and high stability halogen lamps. A ground calibration system of spectral radiances traceable to fixed-point black bodies is used for the pre-flight VNIR and SWIR calibrations.