For the present edition, the book has once again been subjected to substantial revisions. The major additions are in the introductory part which seeks to define the place of legal theory. The important discussions that since the publication of the last edition have taken place with regard to the structure of the legal order, and the revival of the discussions on the relation of law and morality that was provoked by the decision of the House of Lords in the "Ladies directories" case form some of the background for a new chapter (chapter 3) which discusses the concept of law in its relation to the structure of a legal system and the minimum "morality"of a legal order. At the same time, this chapter surveys the principal ethical theories in their relation to legal philosophy. No contemporary jurist can ignore the impact of modern scientific thought on legal theory, as on the social sciences in general. The new chapter (chapter 4) on"Science and Legal Theory" is a very tentative attempt to discuss some of the relevant problems. Lack of professional qualifications in the field of science would have prevented me from presenting these thoughts, had I not had the inestimable benefit of having the chapter checked by my friend and colleague Professor Ernest Nagel, of Columbia and Rockefeller Universities.