Political units, be they kingdoms, empires or states, have never existed in isolation; they have always had to deal with some type of ‘outside’. The nature of these relations constitutes the subject matter of international theory. This course is concerned with various strands of international theory from the late Middle Ages to the present day. It approaches the study of international theory in three steps. While political theorists can readily summon a vast canon of thinkers, the same cannot be said about international theorists. The first section therefore engages in a discussion of the theoretical and methodological questions involved in the historical study of international theory. The second (and longest) section deals with different strands of international theory from the late Middle Ages to the First World War. The third and final section is devoted to contemporary international theory and evaluates the place of these debates within broader historical attempts to make sense of international relations.