This chapter addresses the nature of international organizations and how they are generally theorized as participants in global politics and then reviews the rise of international organizations from a historical perspective, with particular reference to developments from the nineteenth century onwards. It also discusses the major intergovernmental institutions that emerged in the twentieth century and which have played such an important role in shaping global order. The chapter briefly looks at the League of Nations but most attention is given to its successor, the United Nations (UN), and its various appendages. It then examines the world of non-governmental organizations (NGOs). Some of these NGOs possess significant status in the global sphere, others have little relevance, and still others pose dangers. Finally, the chapter considers social movements and their relationship to the contemporary world of international organizations along with the idea of global civil society. In reviewing these institutions, actors, and ideas, we should keep in mind that liberal international theory, especially in the form of liberal institutionalism, as well as proponents of international society, regard robust international organizations as essential building blocks of global order.