Since the passage of the Clean Air Act in 1970 and the creation of the U.S. Environmental Protection Agency the same year, the United States has had in place strict federal standards for key air pollutants and strong incentives for the States to meet these standards. The authors examine several such examples. They are drawn from the U.S. experience in the study and mitigation of photooxidants. The examples include issues of scale and the change over time in our understanding of the importance of various scales, the evolving science of biogenic hydrocarbon chemistry and emissions, and the promise of air quality simulation models. Major programs, such as the Southern Oxidants Study (SOS) and the North American Research Strategy for Troposheric Ozone (NARSTO) are discussed. Finally, a conceptual framework for the relationship between the science and air quality policy communities is presented, and the proper role of science in the relationship is discussed.