Through the study of international relations is a conceptual pursuit, it has only been given formal academic recognition this century. This volume reviews how the great thinkers of the past considered relations between political and social units. It then broaches the emergence of a discipline which sought first to explain the state system that had emerged in the 17th century and now struggles to come to grips with many different conceptions of a rapidly transforming world society. International relations "then" looks at the conceptual worlds of 18th and 19th century theorists and practitioners which gave birth to the creative period of institution-building produced by the demand that World War I would be the last. This was the foundation of international relations as an academic discipline. There follows the first of several debates in the field, that between idealism and realism which ended up with a consensus on the latter in the decades surrounding World War II. In the second part of the book the analysis deals with contemporary theorizing - how realism was challenged and then resurged, how world society and structuralist approaches are now well established and how what has been an Anglo-American discipline is poised now to become a genuinely global field. This intellectual history of our efforts to understand international relations provides an introduction for the advanced student in International Relations with some background in the cognate fields of philosophy, law, history and the social sciences.