Despite its centrality to the future of nuclear arms control and nonproliferation, progress toward a fissile cutoff has lost momentum. To regain momentum, and to capture many of the security benefits of a cutoff as soon as possible, groups and countries advocating nuclear nonproliferation and disarmament should press the nuclear-weapon and threshold states to commit themselves publicly to a moratorium on the production of fissile material for weapons. Fissile materials—plutonium and highly enriched uranium—are the fundamental ingredients of all nuclear weapons. They are also the most difficult and expensive part of a nuclear weapon to produce. A global, verified ban on the production of fissile materials for nuclear explosives is therefore an essential part of any comprehensive nuclear disarmament and nonproliferation regime. A cutoff would limit the size of potential nuclear arsenals. It would make reductions irreversible if fissile material were transferred from dismantled weapons and other unsafeguarded stocks to nonweapons use or disposal under international safeguards. A cutoff would also strengthen the nonproliferation regime by opening up nuclear facilities in all states to international inspection.