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We synthesize and review retrospective analyses of federal air quality regulations to examine the contributions of the Clean Air Act (CAA) to the vast air quality improvements seen since 1970. Geographic heterogeneity in stringency affects emissions, public health, compliance costs, and employment. Cap-and-trade has delivered greater emission reductions at lower cost than conventional mandates, yet has fallen short of textbook ideals. Market power also influenced the CAA's benefits and costs. New benefit categories have been identified ex post, but specific technology requirements have not yet been rigorously evaluated. Comparisons of aggregate benefits and costs of the CAA are beyond present capabilities.