The U.S. transportation sector relies almost exclusively on oil; as a result, it causes about a third of the nation's greenhouse gas emissions. Advanced technology vehicles powered by alternative fuels, such as electricity and ethanol, are one way to reduce oil consumption. The federal government set a goal for federal agencies to use plug-in hybrid electric vehicles--vehicles that run on both gasoline and batteries charged by connecting a plug into an electric power source--as they become available at a reasonable cost. This goal is on top of other requirements agencies must meet for conserving energy. In response to a request, GAO examined the (1) potential benefits of plug-ins, (2) factors affecting the availability of plug-ins, and (3) challenges to incorporating plug-ins into the federal fleet. GAO reviewed literature on plug-ins, federal legislation, and agency policies and interviewed federal officials, experts, and industry stakeholders, including auto and battery manufacturers.