A collaboration between a philosopher and an international relations scholar, this work examines the philosophical issues that underlie the theory of international relations. Part I focuses on the dominant theories of Idealism, Realism, and Behavioralism, and Part II examines the international system, the state, bureaucracies, and the individual—four factors commonly assumed to account for international behavior. The authors conclude with a summary of the links between the two forms of analysis and an open-ended assessment of their relative merits which will stimulate further discussion.