As the 113th Congress has considered air quality issues, oversight of Environmental Protection Agency (EPA) regulatory actions has been the main focus. Of particular interest has been EPA's Clean Air Act regulations on emissions of greenhouse gases (GHGs). Reducing GHG emissions to address climate change is a major goal of President Obama, but Congress has been less enthusiastic about it. In the absence of congressional action, the President has directed EPA to promulgate GHG standards using existing authority under the Clean Air Act. This authority has been upheld on at least three occasions by the Supreme Court, but it remains controversial in Congress. EPA's most recent GHG actions have involved power plants. On June 2, 2014, the agency proposed GHG emission standards for existing fossil-fueled (coal, oil, and natural gas) power plants. These plants are the source of one-third of the nation's GHG emissions, so it is difficult to envision a regulatory scheme that reduces the nation's GHG emissions without addressing their contribution. At the same time, affordable and reliable electric power is central to the nation's economy and to the health and well-being of the population. Thus, the effects of the proposed rule on the electric power system are of considerable interest. Even before proposal of the existing power plant standards, the House had passed legislation (H.R. 3826) that would effectively prohibit EPA from promulgating or implementing power plant GHG emission standards. On September 18, 2014, the House passed the same language a second time, in H.R. 2. The Senate has not considered either bill, but it could still be asked to consider similar measures as amendments or riders on other legislation considered likely to pass in the lame duck session. Although it has not finalized the power plant GHG regulations, EPA has implemented permit and Best Available Control Technology requirements for new stationary sources of GHGs under a separate Clean Air Act provision, the Prevention of Significant Deterioration (PSD) program. Minimum thresholds have exempted smaller pollution sources from this program, and few facilities have needed to obtain permits. Nevertheless, EPA's authority to implement these requirements was challenged in court. In a June 23, 2014, decision, the Supreme Court largely upheld EPA's authority. Besides addressing climate change, EPA has taken action on a number of other air pollution regulations, often in response to court actions remanding previous rules. Remanded rules included the Clean Air Interstate Rule (CAIR) and Clean Air Mercury Rule-rules designed to control the long-range transport of sulfur dioxide, nitrogen oxides, and mercury from power plants through cap-and-trade programs. Other remanded rules included hazardous air pollutant standards for boilers and cement kilns. The agency has also developed revisions of several existing air standards-in general, the Clean Air Act mandates that existing standards be reviewed periodically and revised if appropriate. On March 3, 2014, EPA revised regulations that limit the sulfur content of gasoline, in conjunction with tighter ("Tier3") standards for motor vehicle emissions. The agency has also proposed revised standards for wood and pellet stoves and proposed standards for other types of wood heaters for the first time. EPA is also reviewing ambient air quality standards (NAAQS) for ozone. The agency is under court order to complete a proposed revision of the ozone NAAQS by December 1, 2014. NAAQS serve as EPA's definition of clean air for six widespread pollutants, and drive a range of regulatory controls.