As the new presidential term begins, much of the work of securing the world's nuclear stockpiles so that they cannot fall into terrorist hands remains unfinished. Scores of nuclear terrorist opportunities lie in wait in countries all around the world--sites that have enough nuclear material for a bomb and are demonstrably not adequately defended against the threats that terrorists and criminals have already shown they can mount. These insecure caches also represent opportunities for hostile states, because stolen nuclear material could cut years off the time needed to obtain their first bomb. Separated plutonium and highly enriched uranium (HEU), the essential ingredients of nuclear weapons, are too difficult for terrorist groups to produce themselves. If the world's stockpiles of these materials and of nuclear weapons themselves could be effectively secured, nuclear terrorism could be reliably prevented, and hostile states could be blocked from taking advantage of this potential shortcut to the bomb. With effective action now, the danger could be substantially reduced during President Bush's second term. Success would require sustained presidential leadership to overcome the myriad political and bureaucratic obstacles to progress, but it would not require enormous investment or the development of technologies not already in hand. President Bush thus has an historic opportunity to leave, as a lasting legacy, a world in which nuclear terrorism is no longer a principal threat to world security.