A former nuclear weapons designer and head of nuclear weapons research at Los Alamos, Stephen M. Younger delivers an insightful and urgent inquisition on the role of nuclear weapons in the twenty-first century. Does the United States need a massive atomic arsenal in an era of precision bombs and missile defense? Under what circumstances might we use nuclear weapons? And how does the proliferation of nuclear weapons by North Korea and Iran affect America's nuclear policy? With his deep understanding of the technology and politics of nuclear arms, Younger challenges us to join a new debate on the future of the ultimate weapon.