Policy diffusion has become one of the most vibrant areas of research in the study of American state politics, and State Politics & Policy Quarterly ( SPPQ ) has played a leading role in shaping that scholarship. While scholars agree that states routinely look to one another when making policy decisions, questions persist about the mechanisms that drive these processes, the methods best suited to studying them, and the implications for policymaking. This article reviews 25 years of diffusion research published in SPPQ. We classify articles into four categories – those developing theory, advancing methods, applying diffusion insights to substantive policy domains, and diffusion-adjacent studies – and analyze both their content and contributions. Our review highlights the richness of this work, the frequent overlap across categories, and the distinctive advantages of studying diffusion in state politics. We conclude by outlining opportunities for the next generation of research and reaffirming SPPQ ’s role in advancing the field.