This book traces the development of the international society tradition from its origins in Grotius’ On the Law of War and Peace to its crystallization in Bull’s The Anarchical Society. It follows the idea of sociability among peoples as it was presented by Grotius and substantiated by Pufendorf, through the skepticism of Voltaire and Kant, to emerge as humanitarian warfare and human rights in the international liberal movement, ‘world society’ in the 20th century Catholic revival, and common practices and social understandings in the English School in the period of disciplinary development in international relations after the Second World War.