Emissions from mobile sources, such as automobiles and trucks, contribute to air quality degradation and can threaten public health and the environment. Under the Clean Air Act, the Environmental Protection Agency (EPA) regulates these emissions. The act generally allows one set of federal standards for new motor vehicle emissions and pre-empts states from adopting or enforcing their own standards. However, it also authorizes the EPA Administrator to waive this provision to allow the state of California1 to enact and enforce emission standards for new motor vehicles that are as protective, in the aggregate, as federal government standards. Other states may also adopt California's standards if they choose. The waiver provision was added to the Federal Air Quality Act (one of the precursors of the current Clean Air Act) in 1967 because of California's severe air pollution problems and because the state had already established its own emission standards for mobile sources. California has used this waiver provision regularly to establish and enforce standards for vehicle emissions more stringent than those required by federal law. However, California must request a waiver of federal pre-emption and the EPA Administrator must approve it before California or any other state can implement such standards. Since being given this authority, California has requested and been granted waivers more than 50 times. In December 2005, California requested a waiver from EPA to allow it to regulate motor vehicle emissions of greenhouse gases, which are closely linked to global climate change. At the time, EPA was responding to litigation initiated by environmental groups and state and local governments regarding whether greenhouse gas emissions were air pollutants that the agency had authority to regulate under the Clean Air Act. EPA delayed action on California's waiver request pending the outcome of that litigation. On April 2, 2007, the U.S. Supreme Court decided the question in Massachusetts v. EPA by holding that EPA did have the authority to regulate greenhouse gas emissions. Nevertheless, on December 19, 2007, the EPA Administrator announced his intent to deny California's request. The Administrator subsequently formalized the denial in a decision document he signed and EPA published in the Federal Register on March 6, 2008. The decision has received a high level of attention for a number of reasons. For example, it departed in certain respects from EPA's previous waiver determinations--it was the first time that EPA denied a formal waiver request outright, and it also was the first time EPA used the "compelling and extraordinary conditions" criterion in the Clean Air Act as the basis for denying a waiver request. Due to the atypical outcome of EPA's decision regarding California's greenhouse gas waiver request, Congress asked GAO to review the decision to deny the waiver. As agreed with Congressional offices, we focused our work on the process for and outcomes of past waiver requests because the greenhouse gas waiver decision is the subject of ongoing litigation. Thus, we did not seek to examine the basis for the greenhouse gas decision itself or the process EPA used in reviewing this waiver request. This report summarizes the information about prior waiver requests and decisions provided to Congressional staff during our November 21, 2008, briefing.