Transportation conformity is required under the Clean Air Act (CAA) Section 176(c) to ensure that Federally-supported transportation activities are consistent with (“conform to”) the purpose of a State Implementation Plan (SIP). Transportation conformity establishes the framework for improving air quality to protect public health and the environment. Conformity to the purpose of the SIP means Federal Highway Administration (FHWA) and Federal Transit Administration (FTA) funding and approvals are given to highway and transit activities that will not cause new air quality violations, worsen existing air quality violations, or delay timely attainment of the relevant air quality standard, or any interim milestone. This Guide was prepared to help State and local officials understand transportation conformity and how conformity requirements relate to transportation investments in their communities. Specifically, the implications of conformity on metropolitan transportation plans, transportation improvement programs (TIPs), and transportation projects are discussed. The Guide provides overview information on the major elements of the conformity process and provides answers to basic questions. Several exhibits are included in the Guide to illustrate key elements of the conformity process. Appendices are also included that discuss the health effects of pollutants, options to reduce on-road mobile source emissions, and resource agency contacts.