As Congress debates comprehensive climate change legislation, a second line of action is underway in the United States to regulate greenhouse gases (GHGs). The U.S. Environmental Protection Agency (EPA) has begun a series of rulemakings to reduce GHG emissions under existing provisions of the Clean Air Act (CAA). To date, EPA has initiated a number of actions that are required under the U.S. Supreme Court's 2007 decision in Massachusetts v. EPA, including new emissions standards for mobile sources and preconstruction permitting for new and modified major stationary sources under the prevention of significant deterioration (PSD) program. EPA has also signaled that additional regulation may be in store for existing stationary sources under the new source performance standards (NSPS) program, with official notice coming as early as spring 2010. The most likely regulatory pathways are outlined in Table 1: