The main points discussed in the book include the requirements and the mechanism of international law formation, kinds of international customary rules, relation of customary rules to other kinds of rules of international law, the ascertaining of customary international law, and the basis of its binding force. This edition has been updated by considering the most representative practice and literature of the last 30 years. In this connection, some problems are more widely discussed than in the previous edition, eg the custom-generating effect of treaties and international organizations. The book is based on the principal idea that customary international law, being the most direct outcome of the international reality, should be interpreted primarily in the light of this reality. This work should be of interest to students and teachers of law and political science and scholars and practitioners concerned with international legal problems.