This book deals with the history of the theory of international law in the twentieth century. At its center stands the historical reconstruction of the ideas on international law advanced by Hans Kelsen and his most important students. Those ideas arose for the most part in the period between 1916 and 1950. My goal is to develop an overarching approach that explains the specific orientation and inner structure of Kelsen's works on international law against the background of the debates over the theory of international law and legal policy carried on in his day. To that extent, the reconstruction I have undertaken is grounded in a historical perspective on the evolution of the discipline of international law. At the forefront is an examination of the discourses about the method and construction of international law that influenced Kelsen and his students and which were at the same time substantially shaped by them. In the process, however, attention will also be given to nineteenth- and early twentieth-century theoretical approaches to international law that Kelsen encountered before and during the First World War. I will use these theoretical debates to develop a historical approach to explaining the particular orientation and inner structure of the theory of international law articulated by the Austrian jurist Kelsen. The “key” to Kelsen's writings on international law that I offer here can also provide an answer to the question why they were, on the one hand, among the most vehemently criticized approaches to the theory of international law of the twentieth century, and, on the other hand, do not seem to have lost their fascination for scholars even at the beginning of the twenty-first century.