The United States emerged from the 1990s as an unrivaled global “unipolar” state. This extraordinary imbalance has triggered global debate. Governments and peoples around the world are struggling to understand to how an American-centered unipolar system operates—and to respond to it. What is the character of domination in a unipolar distribution? To what extent can a unipolar state translate its formidable capabilities into meaningful influence? Will a unipolar world be built around rules and institutions or be based more on the unilateral exercise of unipolar power? Scholars too are asking these basic questions about unipolarity and international relations theory. The individual contributions develop hypotheses and explore the impact of unipolarity on the behavior of the dominant state, on the reactions of other states, and on the properties of the international system. Collectively, they find that unipolarity does have a profound impact on international politics. International relations under conditions of unipolarity force a rethinking of conventional and received understandings about the operation of the balance of power, the meaning of alliance partnerships, the logic of international economic cooperation, the relationship between power and legitimacy, and the behavior of satisfied and revisionist states.