This article discusses the dynamics between informal groups of states and the UN Security Council. First, I argue that informal groups have proliferated in response to systemic change. Second, these groups serve as a mechanism that allows for exit from structural constraints of the Security Council and voice for stakeholders in a conflict. In effect, they may narrow the operational and participatory gap growing out of the multiple incapacities that prevents the Council from formulating an effective response to crisis situations. Third, the processes of diplomatic problem solving and its collective legitimation have become increasingly decoupled. The former tends to be delegated to informal groups or coalition of states, while the Council provides the latter. I illustrate how these findings affect one's understanding of power, legitimacy, and change in the theory of international relations.This article is the extensively revised version of a paper presented at the Annual Meeting of the Academic Council on the United Nations System in 2003. The project received financial support from the Economic and Social Research Council, United Kingdom (Grant No. R42200024335), and the Centre for International Studies, University of Oxford. The UN Studies Program at Yale University was a frequent host and home over recent years. I would like to express my gratitude for the long-term support and advice of Karl Kaiser, Bruce Russett, James Sutterlin, and especially Neil MacFarlane. I also wish to thank Mats Berdal, Richard Caplan, Sam Daws, Kurt Gaubatz, Marrack Goulding, Jean Krasno, Edward Luck, David Malone, Lisa Martin, James Mayall, Joseph Nye, Adam Roberts, Avi Shlaim, Ngaire Woods, and two anonymous referees for comments and criticism.