This chapter discusses the concept of formal legal rationality by bringing together the conceptions of Max Weber and of Hans Kelsen. The relationship between the thought of Kelsen and that of Weber is certainly complex, but altogether fascinating and rich in lessons for the sociology of law. In large part, Weber focuses his interest in the Sociology of Law on the processes of rationalization which foster the logical coherence and systematization of law. One of the most controversial aspects of the legal and political sociology of Weber rests on the relationship established between the formal rationality of law and the type of legitimacy characteristic of modernity, the belief in the legality of domination. The fundamental divergence between Weber and Kelsen strongly influences the incompatibility of the Kelsenian postulate of the identity of law and State which we have evoked, and the Weberian distinction between State and non-State law.