The federal Clean Air Act (CAA) has been both criticized as a cause of sprawl and praised as a useful tool to curb it. Critics contend that by barring increases in air pollution in cities where the air is unhealthy, the law drives businesses and development to outlying areas, thus increasing sprawl and the air pollution from its attendant motor vehicle travel. This is the basis for claims that the Act can have the perverse and unintended effect of increasing air pollution rather than reducing it. However, the Act’s defenders argue that it actually can deter sprawl by providing an incentive for transit-oriented, compact development, and urban revitalization. This argument credits the CAA, and the conformity provisions in particular, as a factor in spurring new types of urban development that facilitate transit use and pedestrian traffic and reduce automobile dependence. This study attempts to reconcile the contrasting views of the law by examining its application in several major metropolitan areas. The results suggest that the Act does not necessarily divert growth from urban centers and indeed can complement efforts to promote growth in areas with existing infrastructure.