Even as pundits are proclaiming the end of the United States' unipolar moment, scholars are still trying to understand the exact nature of U.S. primacy. If the United States is the most powerful state the world has seen, what constrains or disciplines its security pursuits? In this important book, Brooks and Wohlforth survey the leading schools of thought looking for answers. Their controversial argument is that none of the traditional constraints on powerful states seems to hold under conditions of unipolarity. The balance-of-power mechanism stressed by realists is nowhere to be found. Nor, in their view, do multilateral institutions or concerns about legitimacy act as brakes on U.S. security policy. In a sort of lawyer's brief, Brooks and Wohlforth find each of the prevailing perspectives on international relations inadequate as a guide to a one-superpower world. Many, however, will dispute their claim that the United States is not constrained by the felt costs of a diminished reputation or lost legitimacy. After all, if leaders feel these costs, it is hard for Brooks and Wohlforth to say they do not exist. But the authors are surely correct that scholars should pay more attention to other sorts of constraints -- such as nationalism, insurgency, nuclear proliferation and deterrence, and imperial overstretch. The book's implicit message is that the United States is not so encumbered today that it cannot step forward to reshape world politics. Doing so, however, will ultimately require taking seriously the realist and liberal theories that Brooks and Wohlforth have called into question.