This article seeks to inform current debates on the changing architecture for global governance by cataloguing and suggesting evaluation criteria for alternative multilateral arrangements. Rather than describing a system in crisis, it focuses on the dynamics of change and flexibility in which established intergovernmental organizations are challenged to meet new demands and requirements while accommodating new mandates and members as well as nonstate actors with global reach. A proliferating and fluctuating set of intergovernmental and multi-stakeholder arrangements with more assertive and diverse actors best describes the international operating environment for collective decisionmaking and action across a range of global issues, raising fundamental questions of effectiveness, accountability, legitimacy, and sustainability and posing challenges to the authority of existing IGOs. KEYWORDS: global governance, multilateralism, intergovernmental organizations, global issues, collective action, and international public policy. ********** The world has changed dramatically since the end of World War II when the United Nations, the Bretton Woods institutions, and a set of regional organizations were established to attend to matters of peace and security and economic cooperation. (1) What is now only too well known as accelerated globalization has radically multiplied the numbers of international transactions, actors, and modes of information sharing and communication that were previously dominated by established governments. In addition to the multiplication of countries seeking a voice in international forums, transnational movements of civil society, nongovernmental organizations (NGOs), multinational corporations, and even wealthy individuals are influencing the ways in which international public policy is made and implemented. Through advocacy, lobbying, and direct service provision (and now global terrorism), these nonstate actors are changing perceptions and behavior in fields as diverse as international health, environmental management, peace and security, human rights, and trade. Intergovernmental organizations (IGOs) are themselves under constant pressure to reform, and new hybrid and ad hoc interstate arrangements open avenues to extra-institutional responses, multiplex decisionmaking forums, and forum shopping. While much has been made of an alleged "crisis in multilateralism" in the wake of the Security Council's failure to mediate the 2003 Iraq crisis and the US withdrawal from and abrogation of a number of treaties, such as the International Criminal Court, the Kyoto Protocol, the Anti-Ballistic Missile (ABM) Treaty, and the Biological Weapons Convention Protocol, collective action to address a range of global and transnational problems is in fact highly dynamic and adaptable. Without question, some multilateral institutions, notably the United Nations, face daunting challenges, as evident in Kofi Annan's dramatic invocation to member states to initiate a set of critical reforms: We have come to a fork in the road. This may be a moment no less decisive than 1945 itself, when the United Nations was founded. We must now decide whether it is possible to continue on the basis agreed when the United Nations was first set up, or whether radical changes are needed, to deal with threats ranging from terrorism and weapons of mass destruction to the possibility that some States may act preemptively to respond to threats.... It is not enough to denounce unilateralism, unless we also face up squarely to the concerns that make some states feel uniquely vulnerable, since it is those concerns that drive them to take unilateral action. We must show that those concerns can, and will, be addressed effectively through collective action. (2) Efforts to meet this institutional reform challenge occur in a number of ways, ranging from debates about enlarging the Security Council to make it more representative and "democratic" to the proposed Assemblage of Democratic States or the expansion of the G8 or G-20 both in membership and mandate. …