This paper assesses the capacity of the Association of Southeast Asian Nations (ASEAN) to moderate great-power relations in East Asia, especially in light of recent regional developments that have challenged ASEAN’s traditional modus operandi and its corporate cohesion. The first of three sections argues that capacity emerges not from institutional arrangements but rather the social relationships that give rise to particular institutions, and therefore can only be understood relationally. A number of key relationships are highlighted and explored in the rest of the paper. First, the relationships among regional great powers, which are considered in section two. Second, the relationships among ASEAN states, and between ASEAN states and their own societies, which are considered in section three. The paper's basic argument is that the first set of relationships is essentially what gives ASEAN its capacity to play a wider regional role. However, it also sets profound constraints for what this role can involve in practical terms. The second set of relationships also creates serious and deep constraints that are often not well understood. However, despite the serious limitations on ASEAN’s leadership role, unless the first set of relationships change, this role is likely to continue, regardless of how frustrating or ineffectual it might be.