The 2002 National Security Strategy suggested preventive attacks, diplomacy, deterrence, and other policies as means of curtailing threats presented by the spread of nuclear, biological, and chemical (NBC) weapons to terrorists and rogue states. Dr. Dan Reiter, the author of this External Research Associates Paper, analyzes which mix of these policies might best and most cost-effectively address the NBC threat, with special focus on preventive attacks. The past performances of preventive attacks, diplomacy, deterrence, and other policies as means of curtailing the NBC threat are analyzed. The author's central findings are that preventive attacks are generally unsuccessful at delaying the spread of NBC weapons; that deterrence, especially nuclear deterrence, is highly successful at preventing the use of NBC weapons by states; and that diplomacy has had moderate and perhaps unappreciated success at curtailing the spread of NBC weapons.