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Calibration accuracy and stability are key performance parameters for Earth-observing satellites. They have direct impact on the quality of science data products derived from sensor observations. Meanwhile, calibration consistency between spectral bands or among sensors also plays a critical role when deriving data products using multiple spectral bands and constructing long-term climate data records (CDR) using observations made by different sensors. The Moderate Resolution Imaging Spectroradiometer (MODIS) is a key instrument for the National Aeronautics and Space Administration (NASA) Earth Observing System (EOS) Terra and Aqua missions. To maintain its on-orbit calibration accuracy and monitor its calibration stability, MODIS was designed and built with a set of onboard calibrators (OBCs), which can be operated continuously or on an as needed basis. In addition to regular onboard calibration activities, extensive efforts have been made by MODIS calibration scientists and members of the MODIS Characterization Support Team (MCST) to validate the calibration accuracy of each sensor and to quantify the calibration consistency between both Terra and Aqua MODIS. This paper provides a brief description of MODIS on-orbit calibration methodologies and an overview of recent progress and lessons learned from MODIS calibration intercomparison studies through the use of ground reference sites. Specifically, the simultaneous nadir overpass (SNO) and double difference approaches over invariant reference sites, such as the Dome C, Libya-4 desert, and Lake Tahoe buoy sites, are illustrated. Examples derived from different intercomparison approaches and their applications for different sensors and spectral bands are presented, focusing on examining their calibration differences. Results from the Libya-4 desert site show that Terra and Aqua MODIS 0.65 and 0.85 mu m channels have been well calibrated, with excellent long-term stability of better than 1.0%. Using the SNO approach, the Terra and Aqua MODIS calibration consistency is found to be within 1.5% and 0.6% for the 0.65 and 0.85 mu m channels, respectively, and 0.02 K and 0.04 K for the 11 and 12 mu m channels, respectively.