Terra Moderate Resolution Imaging Spectroradiometer (MODIS) has made continuous global observations for more than six years since its launch in December 1999. MODIS has 36 spectral bands: 20 reflective solar bands (RSBs) with wavelengths from 0.41-2.2 mum and 16 thermal emissive bands with wavelengths from 3.7-14.4 mum. It is a cross-track scanning radiometer that collects data at three nadir spatial resolutions: 0.25 km (2 bands), 0.5 km (5 bands), and 1 km (29 bands). An onboard solar diffuser (SD) and an SD stability monitor (SDSM) are used biweekly for RSB on-orbit radiometric calibration. Another onboard calibrator (OBC), a spectroradiometric calibration assembly, is used periodically to evaluate and monitor RSB spatial and spectral performance. In addition to measurements made using OBCs, lunar observations at nearly identical phase angles are used to track RSB calibration stability. This paper provides an overview of MODIS RSB on-orbit calibration algorithms and operational activities. It discusses sensor characteristics that could impact RSB calibration accuracy and data product quality, including degradation of the SD bidirectional reflectance factor (BRF), degradation of the scan mirror reflectance in the visible spectral region, and changes in operational configuration. The Terra MODIS OBCs have performed well in monitoring SD degradation and tracking changes in RSB response. Band 8 (0.41 mum) has experienced the largest response decrease with an approximate annual rate of 4.5% (mirror side 1). Band 9 (0.44 mum) has an annual response decrease of about 2.3% (mirror side 1). For most RSB bands with wavelengths greater than 0.5 mum, the annual response changes are generally less than 1.0%. Results from the SDSM on-orbit observations show that the SD BRF also has a similar wavelength-dependent degradation, with the largest degradation appearing at the shortest wavelengths. Among the 330 RSB detectors, there are no inoperable detectors, and only a few noisy detectors have appeared postlaunch