This dissertation is a history of nuclear secrecy in the United States, from the Manhattan Project through the "War on Terror. " It covers nearly seven decades of the attempts made to control nuclear technology through the control of knowledge, and looks at the overall dynamics of American secrecy policies as they unfolded over the course of the latter-half of the twentieth century and the beginning of the twenty-first. The project examines how nuclear secrecy served as a focal point for competing ideas about the nature of science, technology, and governance, and was a vital site for understanding the ways in which the idea of knowledge as power has been articulated and re-articulated in the years after Hiroshima and Nagasaki. The chapters attempt to provide a broad framework for periodizing American nuclear secrecy as a non-monolithic, ever-shifting, and always controversial series of practices of information regulation. The dissertation breaks the history of nuclear secrecy into five primary parts. Part I traces the early history of nuclear secrecy from its emergence in the years just before World War II through its massive implementation during the wartime Manhattan Project, emphasizing that most scientific, administrative, and military participants believed that secrecy would be a strictly temporary condition. Part II covers the attempts to address the immediate postwar problem of what to do about nuclear secrecy, as the wartime project was brought into the realm of public discourse. Part III covers the efforts of the U. S. Atomic Energy Commission (AEC) to develop a coherent secrecy policy as it grappled with a fraught domestic and international political scene, and discusses the emergence of a Cold War model of secrecy. Part IV covers a series of major confrontations as the brittleness of the Cold War model became evident over the course of the 1970s, when new historical actors, threats, and public perceptions came to challenge the once-stable regime. Part V, the epilogue and conclusion, looks at the legacy of secrecy as it was viewed in the late Cold War, the immediate post-Cold War, and the beginning of the "War on Terror. "