This report describes EPA’s Hierarchical Bayesian model-generated (HBM) estimates of O3 and PM2.5 concentrations throughout the continental United States during the 2005 calendar year. HBM estimates provide the spatial and temporal variance of O3 and PM2.5, allowing estimation of their concentration values across the U.S., independent of where air quality monitors are physically located. HBM estimates are generated through the statistical ‘fusion’ of measured air quality monitor concentration values and air quality model predicted concentration values from EPA’s Community Multiscale Air Quality (CMAQ) computer model. Information on EPA’s air quality monitors, CMAQ model, and HBM model is included to provide the background and context for understanding the data output presented in this report.