After grappling for two decades with the realities of the post¿Cold War era, the UN Security Council must now meet the challenges of a resurgence of great power rivalry. Reflecting this new environment, The UN Security Council in the 21st Century provides a comprehensive view of the council¿s internal dynamics, its role and relevance in world politics, and its performance in addressing today¿s major security challenges. David M. Malone is under-secretary-general of the United Nations and also rector of the UN University (UNU). Sebastian von Einsiedel is director of the UNU Center for Policy Research. Bruno Stagno Ugarte, most recently executive director of the Security Council Report, now serves as deputy executive director for advocacy at Human Rights Watch.