Unipolarity is a novel condition in world politics, and its effects on international alliances have yet to receive sustained theoretical attention. Tracing its impact requires a careful distinction between the purely structural features common to any unipolar system and the unique characteristics of the current unipole (the United States) or the policies undertaken by particular U.S. leaders (such as George W. Bush). In general, the unipole will enjoy greater freedom of action and be less dependent on allied support, enabling it to rely more readily on ad hoc “coalitions of the willing.” Lesser powers will be concerned about the concentration of power held by the unipole, but they will also face larger barriers to concerted action to contain it. Hard balancing against the unipole will be unlikely—unless the unipole begins a major effort to expand—but lesser powers will engage in soft balancing to contain the latter's influence. Medium powers may pursue alliances with others in order to reduce dependence on the unipole, but weaker states are likely to ally with the unipole in order to use its power against local security challenges. Bandwagoning will remain rare even under unipolarity, but disputes over burden sharing and alliance leadership will continue. Weaker states will prefer multilateral arrangements that enhance their own influence, while the unipole will prefer bilateral or ad hoc coalitions of the willing that it can more readily dominate.