In 1969, political scientist Jack Walker published ‘The Diffusion of Innovations among the American States’ in the American Political Science Review. ‘Walker 1969’ has since become a cornerstone of political science, packed with ideas, conjectures, and suggestions that spawned multiple lines of research in multiple fields. In good Kuhnian fashion, Walker 1969 is important less for the answers it provides than for the questions it raises, inspiring generations of political scientists to use the political, institutional, and policy differences among the states to understand policymaking better. Walker 1969 is the rock on which the modern subfield of state politics scholarship was built, in addition to inspiring copious research into federalism, comparative politics, and international relations. This Element documents the deep and extensive impact of Walker 1969 on the study of policymaking in the US states. In the process, it organizes and analyzes that literature, demonstrating its progress and promise.