Controlling GHGs could follow a two-track approach, with Congress and the Administration pursuing new legal authority (for cap-and-trade, carbon tax, or whatever) at the same time that the Administration, through the Environmental Protection Agency (EPA), exercises existing authority under the Clean Air Act to begin regulation of greenhouse gas emissions. The Administration has made clear that its preference would be for Congress to address the climate issue through new legislation. Nevertheless, it is moving forward on several fronts to define how the Clean Air Act might be used. The key to using the Clean Air Act's authority is for the EPA Administrator to find that GHG emissions are air pollutants that endanger public health or welfare. This report discusses EPA's authority under Title II and provides information regarding the mobile sources that might be regulated under this authority. Among these sources, motor vehicles (passenger cars and light trucks, including SUVs) are assumed to be the most likely initial targets for regulation, both because a petition addressing these sources began EPA's consideration of the endangerment issue, and because these are the most significant GHG emission sources among those covered by Title II. Regulation of GHGs from mobile sources might also lead the agency to establish controls for stationary sources, such as electric power plants. That option, the authority for which would come from different parts of the Clean Air Act, is not addressed in this report.