The hundredth anniversary of the original publication of Eugen Ehrlich's Fundamental Principles of the Sociology of Law is nearly upon us. The book earned high praise from Oliver Wendell Holmes, Roscoe Pound, and Karl Llewellyn as one of the outstanding works of its time. Ehrlich has been identified as an early legal realist, a pioneering figure in legal sociology, and a leading theorist of legal pluralism. In this retrospective review, I explain the strengths and weaknesses of this classic book. Ehrlich articulated an unsurpassed account of dynamic social-legal change, an account that remains fresh and timely today.