This article addresses the influence wielded by the formal leaders of international cooperation—those state or supranational representatives that chair and direct negotiations in the major decision bodies of multilateral organizations and conferences. This is a topic that so far has received limited systematic attention by IR theorists, who have tended to treat bargaining parties as functionally and formally equivalent, leaving little theoretical space for formal leadership. Drawing on rational choice institutionalism, I introduce a theory that develops a coherent argument for the delegation of authority to the chairmanship, the power resources of negotiation chairs, and the influence of formal leaders over outcomes. I assess the explanatory power of this theory through evidence on formal leadership in three alternative organizational settings: the European Union, the General Agreement on Tariffs and Trade/the World Trade Organization, and the United Nations environmental conferences. I find in favor of the chairmanship as a source of independent influence in international cooperation. Formal leaders perform functions of agenda management, brokerage, and representation that make it more likely for negotiations to succeed, and possess privileged resources that may enable them to steer negotiations toward the agreements they most prefer.