This chapter introduces readers to the chapters to follow by introducing them to the major theoretical themes of Kelsen's work. After a quick review of Kelsen's biography and the reception of his work in United States, the chapter moves on to a discussion of significant aspects of: the pure theory of law; Kelsen's doctrine of the unity of law and international legal monism; and Kelsen's views on justice and democracy. The chapter concludes with a brief review of Kelsen's most recent book, Secular Religion.