Since its inception six decades ago, the RAND Corporation has been one of the key institutional homes for the study of deterrence. Never a well-loved concept in the United States, deterrence lost any luster it held after the Cold War. The 2002 U.S. national-security strategy proclaimed deterrence's irrelevance for most future national-security challenges. However, the 2006 version of this strategy reversed this move, recognizing that deterrence will be as indispensable for the "long war" as it was for the Cold War. This book examines these six decades of research for lessons relevant to the current and future strategic environments. Among its conclusions are that U.S. domestic politics inevitably requires some considerable reliance on deterrence and that deterrence remains relevant to most of the threats the United States is likely to face, from near-peer competitors to regional states of concern and even to many terrorist organizations. It also makes specific recommendations about policies and force structures the United States should pursue to maximize its deterrent capabilities.