This article analyzes existing scholarship on the diffusion of public policies among the American states, focusing on recent developments in this line of research and suggesting several potential avenues for future work. The analysis is organized around three fundamental questions. First, why does policy diffusion occur? Answering this question will require scholars to devote more attention to concepts such as imitation, emulation, and competition. Second, which political forces facilitate or impede policy diffusion? Answering this question will require scholars to devote more attention to the causal mechanisms at work when states adopt policies like those of other states. Third, what is being diffused? Answering this question will require scholars to think more carefully about the content of public policy, both as an outcome to be explained and as a factor that itself affects the diffusion process.