Theories of foreign policy are united by a concern with the way in which states understand and respond to the world around them. Beyond this broad focus there are, however, multiple perspectives on how questions should be asked and analysis developed, which is to say that theories rely upon a set of ontological assumptions and make a series of epistemological choices. Comprehending the deeper foundations of foreign policy theories and their research agendas and where and why they differ requires, therefore, a thorough consideration of questions of ontology and epistemology.