Hans Kelsen's legal philosophy and legal theory is regarded by many in the field as the most influential theory in this century. This volume makes available some of the best work extant on Kelsens' theory, including papers newly translated into English. It covers topics such as competing philosophical positons on the nature of law, legal validity, legal powers, and the unity of municipal and international law, as well as shedding light on Kelsen's intellectual milieu and his intellectual debts.