Decisionmakers matching wits with an adversary want intelligence--good, relevant information to help them win. Intelligence can gain these advantages through directed research and analysis, agile collection, and the timely use of guile and theft. Counterintelligence is the art and practice of defeating these endeavors. Its purpose is the same as that of positive intelligence--to gain advantage--but it does so by exploiting, disrupting, denying or manipulating the intelligence activities of others. The tools of counterintelligence include security systems, deception, and disguise: vaults, mirrors and masks. The U.S. is a prime target of intelligence activity by foreign states and non-state groups because of its status as the world's only superpower. This book has brought together top practitioners and scholars to explain the importance of counterintelligence today and to explore the causes of U.S. counterintelligence weaknesses. The contributors stress the importance of developing a sound strategic vision in order to improve U.S. counterintelligence, and they emphasize the challenges posed by technological change, confused purposes, political culture, and bureaucratic rigidity. Vaults, Mirrors, and Masks make a convincing case that robust counterintelligence is vital to ensure America's security.