Two major mysteries stand at the foundation of world politics: Why do nations go to fight, and why do they cooperate? One widespread theory (realism approach) is presenting conflict as a normal state in international affairs. Another viewpoint perspective (liberalism approach) characterizes cooperation as the norm. This introductory course is designed to give to students the basic knowledge required to understand and analyze international relations and the main international organizations. Therefore, this course is about the role of the international organizations in institutionalizing cooperation in the international system. The students will begin the course by examining some theoretical issues necessary for understanding international relations. From a theoretical perspective, we will explain the realist, liberal, and constructivist theories. Moreover, we will compare and contrast the main theories of international relations. This is important for providing to students the diagnostic tools required for understanding and studying the main international organizations. During the course we will examine the historical origins of the main international organizations, the way how they are structured and their functions. Furthermore, we will analyze the international and domestic actors that impact their actions and efficiency. After analyzing the goals, achievements and the failures of the League of Nations we will investigate in details the main international organizations, which were created after the Second World War. By the end of the course the students will possess the main concepts and the analytical tools which are needed to understand and analyze the behavior and relationship among the main actors in the international system. The final goal will be to cultivate to students a solid understanding related to the functioning of the international organizations.