This Article examines the central role that knowledge plays in determining the Fourth Amendment’s scope. What people know about surveillance practices or new technologies often shapes the “reasonable expectations of privacy” that define the Fourth Amendment’s boundaries. From early decisions dealing with automobile searches to recent cases involving advanced information technologies, courts have relied on assessments of knowledge in a wide variety of Fourth Amendment contexts. Yet the analysis of knowledge in Fourth Amendment law is rarely if ever studied on its own. This Article fills that gap. It starts by identifying the characteristics of Fourth Amendment knowledge. It finds, for instance, that courts typically look to societal knowledge rather than individual knowledge, allowing them to establish broad precedents to govern police behavior. The Article then draws on communications scholarship and research on the spread of innovations to identify conceptual problems inherent in assessing societal knowledge. It also uses original empirical evidence to evaluate courts’ claims regarding societal knowledge in a variety of important cases. And it contends that a knowledge-based Fourth Amendment will shrink and weaken over time as public awareness of new technologies and threats to privacy continues to grow. In light of these findings, the Article proposes that the knowledge inquiry in Fourth Amendment law, and the reasonable expectation of privacy test with which it is intertwined, be replaced with a legal regime better able to adjust to technological and social change. The Article offers two alternatives, one based on existing laws and property concepts, and the other based on direct normative balancing of the benefits and harms of new surveillance practices. It analyzes the strengths and weaknesses of these alternatives, with the goal of developing a Fourth Amendment regime that can effectively protect privacy in novel technological and social contexts.