EPA's Guideline on Air Quality Models ("Guideline") addresses the regulatory application of air quality models for assessing criteria pollutants under the Clean Air Act. In today's action we promulgate several additions and changes to the Guideline. We adopt a new dispersion model, CALPUFF, in appendix A of the Guideline. CALPUFF becomes the preferred technique for assessing long range transport of pollutants and their impacts on Federal Class I areas. Action on AERMOD and the Emissions and Dispersion Modeling System (EDMS) is deferred. We make various editorial changes to update and reorganize information, and remove obsolete models.