The Clean Air Act requires the United States Environmental Protection Agency to promulgate rules that ensure federal actions conform to applicable state implementation plans (SIPs). Federal agencies are responsible for determining if proposed actions conform to the applicable SIP. The General Conformity Rule applies to federal actions occurring in air basins designated as nonattainment or maintenance for criteria pollutants. Federal actions in attainment areas are not subject to the Rule. Detailed emissions calculations are performed for a project. The results are compared to threshold (de minimis) levels and evaluated for regional significance to determine whether a full conformity determination must be performed for the action. If a full conformity determination is required, the general conformity procedure must be completed prior to commencing the federal action. © 2005 Wiley Periodicals, Inc.