This paper argues that libertarian political philosophers, including Robert Nozick, have erred in neglecting the problem of political obligation and that they ought to embrace an actual consent theory of political obligation and state legitimacy. It argues as well that if they followed this recommendation, their position on the subject would be correct. I identify the tension in libertarian (and especially Nozick's) thought between its minimalist and its consensualist strains and argue that, on libertarianism's own terms, the consensualist strain ought to prevail. I then describe the form of the consent theory that I recommend to libertarians. The paper concludes with an extended defense of this form of consent theory against contemporary liberal-egalitarian criticisms of it (both explicit and implicit), including those of Dworkin, Rawls, and their followers.