This article emerged from the work of the Carnegie Commission on Preventing Deadly Conflict, Washington, DC. The commission addressed several fundamental questions: (1) What are the problems posed by deadly conflict, and why is outside help often necessary to deal with these problems? (2) How can disputes be resolved peaceably? (3) Which strategies work best? (4) Who can do what to implement these preventive strategies? Borrowing from the model of preventive medicine, the commission detailed a repertoire of the most promising political, economic, military, and social tools and strategies that can be mobilized by the international community to assist vulnerable societies in the development of sustainable and equitable arrangements for managing diversity and resolving disputes peacefully. From a comparative examination of intransigent and destructive intergroup conflicts, the commission found that the failure to prevent conflict is most often not a failure of foreknowledge or capacity but of political will. Effective political leadership is often the critical variable for successful prevention. This article seeks to illustrate how the social and behavioral sciences may be usefully applied to the problems encountered by leaders when confronted by the challenges of preventing deadly conflict.