For more than sixty years, since Nagasaki in 1945, no nuclear weapons have been exploded in anger, despite several wars in which one side possessed nuclear weapons. The taboo is an asset to be preserved. New nuclear weapon states should recognize that the weapons have proven useful for deterrence; any other use will almost certainly bring universal opprobrium. Certain responsibilities will accrue to any new nuclear-weapon states: security against accident, sabotage, or unauthorized use. The United States was slow to recognize the need for such security, as it was slow to recognize the crucial importance of designing weapons safe from attack. Perhaps China, a mature nuclear-weapon state, is in a strong position to provide guidance to any nations contemplating nuclear weapons.