On November 26, 2014, the Environmental Protection Agency (EPA) announced proposed revisions to the National Ambient Air Quality Standards (NAAQS) for ground-level ozone. If finalized, the proposal would set more stringent standards, lowering both the primary (health-based) and secondary (welfare-based) standards from the current 75 parts per billion (ppb) to somewhere in a range of 65 to 70 ppb. This report discusses the standard-setting process, the specifics of the current and past reviews, and issues raised by the proposal. NAAQS are standards for outdoor (ambient) air that are intended to protect public health and welfare from harmful concentrations of pollution. If EPA changes the primary standard for ozone to a lower level, it would be concluding that protecting public health requires lower concentrations of ozone pollution than were previously judged to be safe. In high enough concentrations, ozone aggravates heart and lung diseases and may contribute to premature death. Ozone also can have negative effects on forests and crop yields, which the secondary (welfare-based) NAAQS is intended to protect. NAAQS do not directly limit emissions of a pollutant; rather, they set in motion a long process in which states and EPA identify areas that do not meet the standards, and states prepare implementation plans to demonstrate how emissions will be lowered sufficiently to reach attainment. Ground-level ozone, or “smog,” is a widespread pollutant: as of July 2014, 123 million people (40% of the U.S. population) lived in areas classified “nonattainment” for the current 75 ppb ozone NAAQS. A more stringent standard might affect more areas. If the nonattainment designations were made using current data, 358 counties would be in nonattainment with a 70 ppb NAAQS (rather than 155 counties at 75 ppb); at 65 ppb, 558 counties would have monitors showing nonattainment. Emission sources in these areas might have to adopt more stringent controls. EPA maintains that most areas will be able to reach attainment of the new standards—whether at 65 or 70 ppb—as a result of already promulgated regulations for gasoline, autos, power plants, and other sources of emissions. Thus, the agency's estimates of the cost of NAAQS compliance are substantially lower than many earlier estimates. EPA estimates the cost of meeting a 70 ppb standard in all states except California at $3.9 billion annually in 2025; the cost of meeting a 65 ppb standard in the same states is estimated at $15 billion annually. Because most areas in California would have until the 2030s to reach attainment, EPA provided separate cost estimates for California ($0.80 billion to $1.6 billion annually in 2038). EPA's cost estimates are substantially less than one from the National Association of Manufacturers that was widely circulated before the release of EPA's proposal.