Moderate Resolution Imaging Spectroradiometer (MODIS) is a key instrument for NASA’s Earth Observing System (EOS), currently operated on-board the Terra spacecraft launched in December 1999 and Aqua spacecraft launched in May 2002. MODIS was developed with improvements over its heritage sensors in terms of its spectral, spatial, and temporal coverage. It makes observations in 36 spectral bands with wavelengths from visible (VIS) to long-wave infrared (LWIR) and at three spatial resolutions (250m, 500m, and 1000m at nadir) with a complete global coverage in less than two days. Since launch, more than 40 science data products have been continuously generated and extensively used by scientists worldwide for their studies of the Earth system of land, oceans, and atmosphere. In order to maintain its on-orbit calibration and science data product quality, MODIS was built and operated with a comprehensive set of on-board calibrators (OBCs), which include 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). This talk provides an overview of MODIS on-orbit calibration methodologies, activities, and efforts made by the MODIS Characterization Support Team at NASA/GSFC. It illustrates examples of Terra and Aqua MODIS radiometric, spatial, and spectral performance derived from their multi-year on-orbit observations. Discussions will focus on, but not limited to, the use of sensor OBCs. Other approaches and activities developed to support MODIS on-orbit calibration and characterization, such as using regularly scheduled lunar observations and carefully selected ground targets, will also be presented.