This chapter illustrates the deep structure of the Kelsenian approach to international law from an intellectual history perspective. Hans Kelsen (1881–1973) was a Viennese law professor in between the two world wars, who is seen by many as one of the most outstanding, if not the most outstanding, jurist of the twentieth century. Therefore studying the Kelsenian approach includes the political, doctrinal, and philosophical context in which Kelsen developed his fundamental critique of the then-prevailing German international law theory. Furthermore, the chapter reveals the subversive and revolutionary force of Kelsen’s critical methodology with a couple of examples, concluding with a few words on how German international legal scholarship dealt with Kelsen’s legacy after the Second World War.