The ``Guideline on Air Quality Models (Revised)'' (hereinafter, the ``Guideline''), as modified by supplement A (1987) and supplement B (1993), sets forth air quality models and guidance for estimating the air quality impacts of sources and for specifying emission limits for them. The Guideline, codified as appendix W to 40 CFR part 51, is referenced in the PSD (Prevention of Significant Deterioration) regulations and is applied to SIP revisions for existing sources and to all new source reviews. On November 28, 1994 EPA issued a Notice of Proposed Rulemaking to augment the final rule that was published on July 20, 1993. Today EPA takes final action that makes several additions and changes as supplement C to the Guideline. Supplement C does the following: incorporates improved algorithms for treatment of area sources and dry deposition in the Industrial Source Complex (ISC) model, adopts a solar radiation/delta-T (SRDT) method for estimating atmospheric stability categories, adopts a new screening approach for assessing annual NO<INF>2 impacts, and adds SLAB and HGSYSTEM as alternative models. This action is responsive to public comments received. Adoption of these new and refined modeling techniques and associated guidance should significantly improve the technical basis for impact assessment of air pollution sources.