This paper assesses the institutional constraints on the effectiveness of the United Nations over the course of its existence, especially in relation to its central mission to promote international peace and security. Only passing attention is accorded to the Bretton Woods institutions. The paper considers these constraints both in relation to the global setting during the cold war, and in the decade since the fall of the Berlin Wall. It also suggests the limitations that derive from the state-centric orientations of most governmental elites, especially those associated with leading states. At the same time, the paper discusses the achievements of the United Nations, some of which were not anticipated at the time of its inception. The degree to which the Organization has achieved and retained virtually universal membership despite the many sharp tensions among its members is itself an impressive confirmation of the worth, and possibly the necessity, of the UN. It also contrasts with the inability of its predecessor, the League of Nations, to obtain a similar level of participation. The paper concludes with a consideration of reforms in process and institutional arrangement that would make the UN more effective in meeting the main world order challenges of the twenty-first century. At the moment, the prospects for realizing such reforms are not promising, but the pressures of world events could change this outlook rather quickly and unexpectedly.