The appalling events of September 11, 2001, require a major international initiative to strengthen security for nuclear materials and facilities worldwide, and to put stringent security standards in place. This paper recommends a range of specific steps to upgrade security at individual facilities and strengthen national and international standards, with the goal of building a world in which all weapons-usable nuclear material is secure and accounted for, and all nuclear facilities are secured from sabotage, with sufficient transparency that the international community can have confidence that this is the case.