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Abstract: MODIS is a major instrument for NASA’s Earth Observing System (EOS) missions. It is currently operating on-board the EOS Terra and Aqua spacecraft, launched in December 1999 and May 2002, respectively. The MODIS instrument was developed with improvements over heritage sensors in terms of its spectral, spatial, and temporal resolutions, and with more stringent calibration requirements. It makes observations in 36 spectral bands covering wavelengths from 0.41 to 14.4μm and at three nadir spatial resolutions: 250m, 500m, and 1km. Together Terra and Aqua MODIS have produced more than 10 years of global data sets that have significantly helped scientists worldwide to better understand the Earth as an interacting system and the impacts on this system due to human related activities. In order to maintain on-orbit calibration and data product quality, MODIS was built with a complete set of on-board calibrators (OBCs), including a solar diffuser (SD), a solar diffuser stability monitor (SDSM), a blackbody (BB), a deep space view (SV) port, and a spectro-radiometric calibration assembly (SRCA). In this paper, we provide a brief description of both Terra and Aqua MODIS on-orbit operation and calibration activities and present results of on-orbit radiometric, spatial, and spectral characterization. Examples of short-term stability and long-term response changes are illustrated using observations made with the on-board calibrators. On-orbit performance parameters, including detectors noise characterization, are also compared to pre-launch design specifications. [Copyright &y& Elsevier]