The diffusion of ideas and practices is fundamental to our national way of life. In areas from technology to commerce to religion, Americans presume that good ideas will flourish. But in a country built on such presumptions, the diffusion of policy ideas is poorly understood. On issues from abortion and capital punishment to education and the environment, new policies spread from one state or locality to another. Yet, neither party operatives nor policymakers nor political scientists really understand the routes of transmission, or the rules governing them. Karch's innovative and engaging book enables us to appreciate the sources of dynamism in American politics.