The quality of the aerosol optical thickness (AOT) data retrieved operationally from the Moderate Resolution Imaging Spectroradiometer (MODIS) sensors aboard the Terra and Aqua satellites, over land, and over ocean from 2000 to 2003 (Aqua only from June 2002) were evaluated thoroughly. Terra-MODIS versions 3 and 4 data (T003 and T004) and Aqua-MODIS version 3 data (A003) were independently and comparatively evaluated with collocated AOT from ground-based Aerosol Robotic Network (AERONET) Sun photometers. At 550 nm wavelength, 67.3%, 55.0%, and 55.5% of AOT from T003, T004, and A003, respectively, meet the prespecified accuracy conditions of ¿(0.05 + 0.2aot) over land, while about 63.3%, 59.4%, and 62.2% fall within the more stringent range of ¿(0.03 + 0.05aot) over ocean. However, when based on equal standards of comparison and regression analysis, aerosol retrievals are much more accurate over ocean than over land. Analysis of MODIS full regional AOT averages from 12 land and 6 oceanic regions shows that aerosol loading exhibits an annual cycle in almost every region, with the exception of very remote oceanic regions such as the central Pacific. On the basis of regional monthly averages, west Africa, China, and India show the highest peak monthly mean AOT value of ~0.7 at 550 nm, while the highest over-ocean aerosol loading occurs over the Mediterranean and Mid-Atlantic oceans, with a regional monthly peak of ~0.35, which is half of the peak over land. The magnitude of day-to-day variation between morning (Terra) and afternoon (Aqua) AOT varies from region to region and increases with aerosol loading for any given region. However, none of the regions examined show any consistent regional trend in morning-to-afternoon aerosol loading, all showing almost equal likelihood of increase or decrease from morning to afternoon.