The post-Cold War era saw explosive growth in international organizations and interstate cooperation. Cooperation between states within and institution, however, is only one option that states have to address foreign policy problems. They also have choices to cooperate outside of institutions, to act unilaterally, to do nothing, or even to use a different organization. This dissertation closes the gap between studies of characteristics that facilitate cooperation and studies of enforcement of completed agreements by examining when states choose ;;cooperate’ instead of other options. It then proceeds to consider how states choose between the institutions available to them as fora for cooperation.I argue that two factors drive the choice between these foreign policy options: the existence of consensus around a state’s desired outcome, and the existence of capacity in the selected forum to achieve a successful outcome. Consensus is the necessary convergence of preferences for action; capacity is the ability to execute the desired action. In the absence of consensus, states that have sufficient capacity to act independently and achieve their ideal points may do so. Fora in which consensus exists but capacity does not will produce declarations or other non-resource-intensive responses. Where both consensus and capacity exist, states can choose to act within international organizations. I test this argument in the context of European foreign policy cooperation in the European Union, North Atlantic Treaty Organization, Organization for Security and Cooperation in Europe, and Council of Europe. I use a sample of 300 randomly chosen international events to examine all possible outcomes – regardless of whether cooperation occurred – spanning the period 1994 to 2003. Quantitative chapters studying cooperation in the EU and choice between various institutions and outcomes find fairly strong support for the consensus arguments but are unable to test rigorously the claims about capacity. A qualitative study of events in Albania in early 1997 further examines the capacity claims and how states themselves make choices between foreign policy options.