This chapter surveys how some of the main international relations theories can assist in understanding national security problems. First it identifies the landscape in which national security policymaking takes place, where conventional cleavages between policy arenas—like foreign policy and domestic policy—are increasingly breaking down due to factors relating to globalisation. Second, it then explores specific theories and the ways that they explain, prescribe and even try to predict outcomes. In doing so it focuses on three main approaches to international relations: realism, liberalism and constructivism. Third, the chapter applies these theoretical approaches to three national security problems in order to demonstrate how their core assumptions can produce radically different interpretations and solutions. The chapter concludes with some potential lessons for national security policy planners about how theory might be usefully employed.