With increasing efforts on data fusion and long-term climate data records (CDR) using observations made by multiple sensors on the same or different platforms, sensor cross-calibration has become increasingly important. It is known that the uncertainty of climate models or science data records depends not only on the calibration quality of individual sensors but also on their calibration consistency. This paper provides an overview of inter-comparison methodologies applied by the MODIS Characterization Support Team (MCST) at NASA/GSFC for the studies of Terra and Aqua MODIS on-orbit calibration consistency. Improved over heritage sensors, MODIS was built with a set of on-board calibrators (OBC) that include a blackbody (BB), a space view (SV) port, a solar diffuser (SD), and a solar diffuser stability monitor (SDSM). The BB is primarily used for the thermal emissive bands (TEB) calibration and the SD/SDSM system for the reflective solar bands (RSB) calibration. Detector responses to the SV provide measurements for the instrument background. Although instrument design requirements and calibration approaches are nearly identical for both Terra and Aqua MODIS and they all went through extensive and similar pre-launch calibration and characterization activities, their on-orbit calibration consistency still has to be carefully examined and validated as many science products are generated from observations made by both instruments. Methodologies discussed in this paper include inter-comparison studies using the Moon, a third sensor, and ground targets. Our results show that Terra and Aqua reflective solar bands and thermal emissive bands have been calibrated consistently to within their combined uncertainty requirements. For the 11mm and 12mm bands used for surface temperature measurements, the calibration differences between Terra and Aqua MODIS are less than ±0.15K at scene temperatures from 240-280K and less than ±0.50K at cold scene temperatures from 190 to 230K (before corrections). For most reflective solar bands, their reflectance calibration differences are typically less than ±2%.