This paper describes the methodology developed to monitor the radiometric calibration of NOAA AVHRR data and to normalize succeeding polar orbiters for the International Satellite Cloud Climatology Project (ISCCP). Results are presented for NOAA-7, -8, and -9 Channel 1 (visible) data and briefly for Channel 4 (thermal infrared) in an appendix. The successful normalization of NOAA-8 and NOAA-9 to NOAA-7 allows the ISCCP calibration standard to be maintained over lime. A correction for the degradation of NOAA-9 data and an absolute calibration for the entire ISCCP dataset are presented.