The authors describe the U.S. Environmental Protection Agency's new modeling framework, Models-3, and its first version for air quality called the Community Multiscale Air Quality (CMAQ) model. Models-3/CMAQ is a new generation, state-of-science, comprehensive air quality modeling framework. With a 'one atmosphere' paradigm, it is designed to be capable of addressing holistically and inclusively, the major air quality issues such as photochemical smog, particulate matter, airborne toxics, and Air Quality Related Values (AQRVs) including acidic, nutrient and toxic deposition, as well as visibility. For particulate matter (PM), number and size distributed sulfate, nitrate, organic and aerosol-bound water are predicted on three dimensional grid cells for domains encompassing regional and urban scales. The fundamental features of the aerosol formulations include a bimodal size distribution of particles in the sub-micron range, and aerosols that are internally well mixed. Size dependent dry deposition parameterization, aqueous phase aerosol dynamics, and nucleations are included among the various major atmospheric processes modeled. Considerations for extensions and applications of Models-3/CMAQ are discussed.