The Moderate Resolution Imaging Spectroradiometer (MODIS) onboard the NASA EOS Aqua spacecraft has been in operation for more than three years since its launch in May 2002. MODIS is a multi-spectral cross-track scanning radiometer that has 20 reflective solar bands (RSB) from 0.41 to 2.2μm and 16 thermal emissive bands (TEB) from 3.7 to 14.4μm. It makes continuous observations that can be applied to a wide range of studies of the Earth's environment and climate. The sensor's in-flight calibration and characterization activities (radiometric, spatial, and spectral) are performed using a set of onboard calibrators (OBCs) that include a solar diffuser (SD), a solar diffuser stability monitor (SDSM), a blackbody, and a spectro-radiometric calibration assembly (SRCA). In this paper we present on-orbit performance of the Aqua MODIS onboard calibrators using its calibration data sets. We illustrate use of the SD for RSB calibrations and current trending of SD degradation. For the TEB calibration, we discuss BB temperature noise characterization, its short- and long-term stability, and the use of BB warm-up and cool-down cycles to track key TEB calibration parameters. The results of Aqua MODIS spatial and spectral characterization are also addressed. In general the overall Aqua MODIS on-orbit performance has been stable and satisfactory when compared to its design parameters, performance specifications, and pre-launch determined characteristics. In a number of areas, Aqua MODIS is performing better than its predecessor - Terra MODIS.