This textbook, which is aimed at students, academics and interested individuals (journalists, representatives of state or civil society institutions, NGOs), deals with institutionalized cooperation based on international organizations and addresses the law and the role of these organizations in international law and politics. This topic is highly relevant as we live in disruptive times. This makes international cooperation all the more important in overcoming the numerous challenges that the international system and mankind face. The book begins with a general section that explains the legal and theoretical bases of international organizations as pivotal institutions of cooperation between states. This is followed by a closer look at the most important global and European organizations. At the global level, the UN as the central institution of the state system with its collective security mechanisms deserves special attention. International economic organizations are also of great importance, as they legally constitute and steer the global economy. The WTO is an institution that monitors and, if necessary, enforces compliance by states with the central trade agreements GATT, GATS and TRIPS. The Bretton Woods institutions have special features that make them interesting not only because of their obvious global economic impact, but also from a legal perspective. This includes their structure and mode of operation, which is modelled after private-sector companies. At the European level, the OSCE is a remarkable organization because it is based exclusively on soft law. At the same time, it is a regional collective security organization under Chapter VIII of the UN Charter. The Council of Europe is a traditional organization dedicated to human rights and the rule of law, but it has special institutional characteristics due to its close connection to the European Court of Human Rights. NATO is the prototype of a collective defense organization and has special characteristics in terms of decision-making (unanimity), institutions and cooperation with third countries. The EU is the prototype of a supranational organization as the next higher level of intergovernmental cooperation, which is based on the partial integration of powers originally belonging to the member states. All organizations covered in this book have their own unique characteristics, and together they provide a multi-layered picture of the impact of international organizations in the international system. The book concludes with a chapter on NGOs as institutions of international civil society that play an important role in the development of global strategies and international legal standards.