Observations of adverse effects of air pollution on public health, illustrated by the London smog events in the 1950s, led to legislation in the United States requiring development of federal, state, and local air quality management programs. The implementation of management programs has resulted in significant reductions in air pollutant emissions from stationary and mobile sources and hence their ambient concentrations and associated health risks. Evidence of benefits from improvements in air quality can be identified from studies in which rapid changes in air quality have occurred. Health risk assessment and benefits estimates also can be predictive, resulting in mean estimates of avoided mortality in excess of many thousands of cases per year as a result of implementation of air quality management programs in the United States.