In this action, the Environmental Protection Agency (EPA) promulgates revisions to the Guideline on Air Quality Models ("Guideline"). The Guideline has been incorporated into the EPA's regulations, satisfying a requirement under the Clean Air Act (CAA), for the EPA to specify, with reasonable particularity, models to be used in the Prevention of Significant Deterioration (PSD) program. The Guideline provides EPA-preferred models and other recommended techniques, as well as guidance for their use in predicting ambient concentrations of air pollutants. The EPA is revising the Guideline, including enhancements to the formulation and application of the EPA's near-field dispersion modeling system, AERMOD, and updates to the recommendations for the development of appropriate background concentration for cumulative impact analyses.