This book develops a novel multilevel social contract theory that, in contrast to existing theories in the liberal tradition, does not merely assume a restricted form of reasonable moral pluralism, but is tailored to the conditions of deeply morally pluralistic societies that may be populated by liberal moral agents, nonliberal moral agents, and, according to the traditional understanding of morality, nonmoral agents alike. To develop this theory, the book draws on the history of the social contract tradition, especially the work of Hobbes, Hume, Kant, Rawls, and Gauthier, as well as on the work of some of the critics of this tradition, such as Sen and Gaus. The two-level contractarian theory holds that morality in its best contractarian version for the conditions of deeply morally pluralistic societies entails Humean, Hobbesian, and Kantian moral features. The theory defines the minimal behavioral restrictions that are necessary to ensure, compared to violent conflict resolution, mutually beneficial peaceful long-term cooperation in deeply morally pluralistic societies. The theory minimizes the problem of compliance by maximally respecting the interests of all members of society. Despite its ideal nature, the theory is, in principle, applicable to the real world and, for the conditions described, most promising for securing mutually beneficial peaceful long-term cooperation in a world in which a fully just society, due to moral diversity, is unattainable. If Rawls’ intention was to carry the traditional social contract argument to a higher level of abstraction, then the two-level contractarian theory brings it back down to earth.