This chapter examines some of the competing theories that have been advanced to explain U.S. foreign policy. In trying to explain the foreign policy of the United States, a number of competing theories have been developed by International Relations scholars. Some theories focus on the role of the international system in shaping American foreign policy while others argue that various domestic factors are the driving force. The chapter first considers some of the obstacles to constructing a theory of foreign policy before discussing some of the competing theories of American foreign policy, including defensive realism, offensive realism, liberalism, Marxism, neoclassical realism, and constructivism. The chapter proceeds by reviewing the theoretical debate over the origins of the Cold War and the debate over the most appropriate grand strategy that the United States should follow in the post-Cold War era.