There are two primary reasons why empires are central to our understanding of International Relations (IR). First, the empire has been replaced by juridically equal sovereign territorial states over the past century. Formal empires no longer exist, and only one head of state retains the title of Emperor—Akihito of Japan. The second reason why the study of empire matters to IR is that much of the conventional distinction between hierarchy and anarchy has been subject to various criticisms from a wide array of methodological and political perspectives. In particular, International Political Sociology (IPS) has offered a framework for critical analyses of phenomena such as systemic transformation, international unevenness, and global inequality, or war, violence, and racism in international politics. Since the end of the Cold War, new theorizations of empire have placed empire and imperialism at the center of debates in IR. Contemporary investigations of empire in IR, and IPS in particular, have dwelled on a number of political debates and methodological issues, including the nature of American imperialism, the link between IR and global history, and the relationship between empire and globalization. The category “empire” continues to both illuminate the pertinence of IR to social theory more generally and at the same time highlights the shortcomings of the discipline in addressing the causes and dynamics of global inequality, violence, and uneven development.