To help focus debate about the best use of society's resources, it is important to have estimates of the benefits and costs of further improvements in air quality. Such estimates are developed, with focus primarily on reductions in ground-level ozone resulting from the control of volatile organic compounds; to a lesser extent, particulate control also is considered. Proposed controls are evaluated for both the nation as a whole and for the Los Angeles metropolitan area, where violations of air quality standards are most frequent and severe. Subject to a number of uncertainties, the costs of proposed new controls are found to exceed the benefits, perhaps by a considerable margin.