The geopolitical upheavals at the end of the 20th century and the reformation of the state system have also had consequences for the study of International Relations. While a realistic understanding of politics dominated the field during the East-West conflict, idealistically inspired theories have been on the rise since then. This volume provides a profound overview of the theory and history of the discipline from the establishment of the field in the early 20th century to the current phase of reorientation. However, it is more than just a history of ideas and offers more than a comprehensive view of this branch of science, as it also addresses the historical and political context in which the ideological and methodological changes of the field were embedded, as well as the central theoretical discourses on war and peace in the 20th century.