Until recently, most attention to U.S. climate policy has focused on legislative efforts to introduce a price on carbon through cap and trade. In the absence of such legislation, the Clean Air Act is apotentially potent alternative. Decisions regarding existing stationary sources will have the greatest effect on emissions reductions. The magnitude is uncertain, but plausibly 10 percent reductions in greenhouse gas emissions from 2005 levels could be achieved at moderate costs by 2020. This is comparable to the reductions that would have been achieved under the Waxman-Markey legislation in the domesticeconomy. These measures do not include the switching of fuels, which could yield further reductions. The ultimate cost of regulation under the act hinges on the stringency of standards and the flexibility allowed. A broad-based tradable performance standard is legally plausible and would provide incentives comparable to the proposed legislation, at least in the near term.