Nuclear deterrence and nonproliferation no longer enjoy the broad support they once did during the Cold War. Academics and security experts now question the ability of either to cope or check nuclear rogue states or terrorists. On the one hand, America’s closest allies–e.g., Japan and South Korea–believe American nuclear security guarantees are critical to their survival. If the United States is unwilling to provide Tokyo or Seoul with the assurance they believe they need, would it then not make sense for them to acquire nuclear forces of their own? On the other hand, with more nuclear-armed states and an increased willingness to use them, how likely is it that nuclear deterrence will work? This volume investigates these questions. In it, six experts offer a variety of perspectives to catalyze debate. The result is a rich debate that goes well beyond current scholarship to challenge the very basis of prevailing nonproliferation and security policies.