This chapter describes the different kinds of international organizations (IOs) at play in world politics. It also briefly summarizes five theoretical frameworks - realism, liberalism, Marxism, feminism, and constructivism - that are applied systematically throughout the text to explain and analyze the behavior and policies of IOs in real world cases. IOs were conceived as formal institutions whose principal members are states. IO also refers to the institutions, processes, norms, laws, and regulations that are part and parcel of global governance. Global governance involves making and sustaining the rules and the norms of world order. The chapter also provides an overview of this book. The book examines the liberal theoretical approach to international relations and international organizations. It explores international security and the challenges posed by war and other threats to international peace and security. The book surveys how international organizations address the challenges of environmental protection.