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The Medium Resolution Spectral Imager (MERSI) is a key instrument onboard Fengyun-3 (FY-3), the second generation of polar-orbiting meteorological satellites in China. This paper summarizes the knowledge of MERSI instrument in terms of sensor design, calibration algorithm, prelaunch and on-orbit characterization, and performance verification. The calibration monitoring of its reflective solar bands (RSBs) is primarily conducted using a visible onboard calibrator and found that it has a significant degradation on the order of 10% in its shorter RSB bands (<; 500 nm), with the largest in band 8 of about 20% during the past two years. However, the performance at longer wavelength bands is relatively stable with a change of less than 5%. It is shown that the postlaunch calibration of the two short-wavelength infrared bands has frequent fluctuations because of random jumps in their electronic gains. These results are consistently verified by two kinds of vicarious calibration (VC) methods: China Radiometric Calibration Sites VC and intercalibration using Terra/Moderate Resolution Imaging Spectroradiometer over Dunhuang desert. The overall uncertainty in the MERSI top-of-atmosphere radiance or reflectance is less than 5%. These results provide the important reference and evaluation for the update of the FY-3A/MERSI calibration coefficients.