The Nuclear Security Summit’s focus is too narrow for this role; the International Atomic Energy Agency’s formal mandate is too restricted; the NPT Review Conference meets too irregularly; and the United Nations Security Council’s membership is too limited. The best forum for norm-building may prove to be the G-20, whose members embrace both North and South, account for most of the world’s population, GDP, and all but a handful of its nuclear weapons, and whose heads of government meet regularly.