The MODerate Resolution Imaging Spectroradiometer (MODIS) Flight Model 1 (FM1) was launched on May 4, 2002 onboard the NASA Earth Observing System (EOS) Aqua spacecraft. It has provided more than a year of global data for studies of the Earth’s land, oceans, and atmosphere in support of the science community and public users. To assure the quality of the data and science products, extensive efforts have been made to collect and analyze data on the instrument’s on-orbit performance using its on-board calibrators (OBCs). MODIS has 36 spectral bands: 20 reflective solar bands (RSBs) with wavelengths from 0.41 micrometer to 2.2 micrometers and 16 thermal emissive bands (TEBs) from 3.7 to 14.2 micrometers. For radiometry, the RSBs are calibrated by a solar diffuser (SD) and a solar diffuser stability monitor (SDSM) system and the TEBs by a blackbody (BB). An on-board Spectroradiometric Calibration Assembly (SRCA) is used for the instrument’s spectral (RSBs only) and spatial (all 36 bands) characterization. Using the first year’s calibration data sets, this paper presents Aqua MODIS on-orbit performance in three areas: radiometric, spatial, and spectral. Comparisons with the sensor’s specifications and with the performance of its predecessor, Terra MODIS (launched in December 1999), are discussed. Excluding a few problems identified pre-launch, such as non-functional detectors in the 1.6 micrometers band and the out of specification performance for the band to band registration (BBR), the on-orbit observations and analyses show that Aqua MODIS has been performing according to its design characteristics.