Although air quality has improved substantially in the United States in the 40 years of EPA's Clean Air Act regulation, many issues remain unresolved, and, in recent months, members of Congress from both parties have raised questions regarding the cost-effectiveness of, and authority for, EPA actions. This report focuses on three general areas of likely interest to the 112th Congress: greenhouse gas (GHG) regulations, emissions from power plants (including interstate pollution and mercury emissions), and air quality standards.