Epidemiologic and health impact studies of fine particulate matter (PM2.5) are limited by the lack of monitoring data, especially in developing countries. Satellite observations offer potentially valuable global information about PM2.5 concentrations. Van Donkelaar et al. mapped global ground-level PM2.5 concentrations using total-column aerosol optical depth (AOD) from the Moderate Resolution Imaging Spectroradiometer (MODIS) and Multiangle Imaging Spectroradiometer (MISR) satellite instruments and coincident aerosol vertical profiles from the GEOS-Chem global chemical transport model. The authors report a global population-weighted geometric mean PM2.5 concentration of 20 µg/m3. The World Health Organization Air Quality PM2.5 Interim Target-1 was exceeded for 38% and 50% of the population over central and eastern Asia, respectively. Annual mean PM2.5 concentrations exceeded 80 µg/m3 over Eastern China. Comparison of the satellite-derived estimate with ground-based in situ measurements indicated significant spatial agreement with North American measurements and with noncoincident measurements elsewhere. The authors conclude that satellite-derived total-column AOD, when combined with an aerosol transport model, can provide estimates of global long-term average PM2.5 concentrations.