Local policy adoptions provide an excellent opportunity to test among potential mechanisms of policy diffusion. By examining three types of antismoking policy choices by the 675 largest U.S. cities between 1975 and 2000, we uncover robust patterns of policy diffusion, yielding three key findings. First, we distinguish among and find evidence for four mechanisms of policy diffusion: learning from earlier adopters, economic competition among proximate cities, imitation of larger cities, and coercion by state governments. Second, we find a temporal component to these effects, with imitation being a more short-lived diffusion process than the others. Third, we show that these mechanisms are conditional, with larger cities being better able to learn from others, less fearful of economic spillovers, and less likely to rely on imitation.