In many ways, the approach described in this book is a throwback to an older approach to the study of public-policy innovation and diffusion. It follows the pioneering work of Walker (1969), Gray (1973), and Savage (1978) by going back to a large-N comparative analysis of policy innovations spanning more than a century. This stands in stark contrast to the recent trend toward intensive analysis of single cases intended to illustrate the processes contributing to the diffusion of innovations. These single case studies serve well for highlighting specific decision-making processes or for commenting on an anomalous case in which an innovation's diffusion has deviated from the expected pattern. It is when such deviations occur that the limitations of the case-study approach become apparent, for this approach presumes that a single pattern is dominant and that deviations are exceptional. The work presented in this book has taken a more expansive approach. Though possibly sacrificing some of the detail and precision of single case studies, this comparative orientation suggests a broader range of diffusion patterns of emerging innovations in America. It provides insights into how differences across historical eras, policy domains, interest-group carriers, and policy targets shape the pattern of diffusion. What has been sacrificed in detail is offset by gains in generalizability. The approach taken here is empirical. It involved examining a large set of innovations, first to evaluate how well the conventional explanation for the diffusion of innovations matched the actual record, and then to identify the triggers and causes of different patterns of diffusion.