This chapter describes the basic features of realist theory, including its emphasis on the implications of international anarchy and the importance of power. The chapter then explores major divisions within the realist family, and their implications for states’ security policies and war. The most fundamental division is between structural realism—which focuses on the impact of the international system on states’ decisions—and motivational realism—which focuses on the impact of variation in states’ motives. Also important is the ongoing debate within structural realism—between Kenneth Waltz’s structural realism, offensive realism, and defensive realism. The first two of these find that international structure generates a strong tendency towards competitive policies, while defensive realism finds that cooperation is, under some conditions, a state’s best strategy for achieving security. The chapter illustrates how these different arguments result in divergent predictions for how China’s continuing economic growth is likely to influence international security.