This chapter surveys the existing approaches to studying IOs, and discusses our public policy approach. It describes IOs as institutions that are defined by formal and informal rules, by practices and sets of expectations that shape the way those involved in IOs’ activities work. Rather than accepting the traditional proposition that member states decide, the chapter argues that we need to go inside the organization to examine how all the actors perceive their roles, interpret their responsibilities, and interact with each other. It identifies three groups of actors—state representatives, heads of IOs, and secretariats—and discusses their strength, advantages, and levers in IO operations. It particularly highlights the impact of organizational structure, history, and culture on actors’ behaviour and examines their powers of persuasion in a comparative study across six IOs.