EPA is establishing carbon dioxide (CO2) emission standards for existing electric generating units (EGUs) under §111(d) of the Clean Air Act (CAA). The prospect of undertaking such a significant regulatory program under the authority of a little-used provision of the law has generated a number of questions about what EPA may and may not do in shaping this new regulatory policy. Although the CAA can be read to authorize EPA to require comprehensive, systemwide emission reductions from EGUs, the fact that so many fundamental legal questions about the scope of EPA’s authority have not yet been conclusively resolved by the courts introduces a level of legal uncertainty that has seldom been seen in the Agency’s 40-plus year history of regulating air pollution.