Hans Kelsen is considered to be one of the foremost legal theorists and philosophers of the twentieth century. His writing made an important contribution to many areas, especially those of legal theory and international law. Over a number of decades, he developed an important legal theory which found its first complete exposition in Reine Rechtslehre, or Pure Theory of Law, the first edition of which was published in Vienna in 1934. This is the first English translation of that work. It covers such topics as law and morality, the legal system and its hierarchical structure, the identity of law and state, and international law.