In 2003, Congress established a program to reduce airport ground emissions at commercial service airports in areas failing to meet or maintain air quality standards. The Federal Aviation Administration (FAA) administers the Voluntary Airport Low Emissions (VALE) Program and oversees the program's two sources of funding: Airport Improvement Program (AIP) federal grants or Passenger Facility Charges (PFC), which airports can collect from passengers. Participating airports also receive credits for the emission reductions achieved through VALE projects in accordance with the law and guidance. Airports can use these credits to offset emissions resulting from development projects to comply with federal Clean Air Act requirements. GAO examined (1) how the VALE program has been implemented, including airport participation levels, types of projects, and program expenditures, and (2) the outcomes attributable to the VALE program. To do this, GAO reviewed FAA data on VALE projects for all nine participating airports; visited two of these airports; obtained information from the remaining seven participating airports and four nonparticipating airports; and interviewed officials from FAA, Environmental Protection Agency (EPA), and airport associations.