Laws and regulations are ubiquitous, touching on many aspects of individual and corporate behavior. But under what conditions are laws and rules actually effective? A huge amount of recent work in political science, sociology, economics, criminology, law, and psychology, among other disciplines, deals with this question. But these fields rarely inform one another, leaving the state of research disjointed and disorganized. Lawrence M. Friedman finds order in this cacophony. Impact gathers recent findings into one overarching analysis and lays the groundwork for a cohesive body of work in what Friedman labels "impact studies." The first important factor that has a bearing on impact is communication. A rule or law has no effect if it never reaches its intended audience. The public's fund of legal knowledge, the clarity of the law, and the presence of information brokers all influence the flow of information from lawmakers to citizens. After a law is communicated, subjects sometimes comply, sometimes resist, and sometimes adjust or evade. Three clusters of motives help shape which reaction will prevail: first, rewards and punishments; second, peer group influences; and third, issues of conscience, legitimacy, and morality. When all of these factors move in the same direction, law can have a powerful impact; when they conflict, the outcome is sometimes unpredictable.