Is there a general obligation to obey the laws of a reasonably just polity? Is there any justification for imposing suffering, in the form of punishment, on those who break the law? Political and legal philosophers have long debated these vexing questions, but the debates typically have taken up each question in isolation. Playing Fair, however, treats the two questions as intertwined and provides affirmative answers to both—answers grounded, in both cases, in the principle of fair play. According to this principle, those who are engaged in a mutually beneficial cooperative practice or enterprise have a duty to the cooperating participants to bear a fair share of the burdens of the practice. Applied to the political order, the principle holds that a reasonably just polity is a cooperative enterprise whose members receive benefits from the rule of law only because other members obey the law even when they find obedience burdensome. The members of a reasonably just polity thus have a political obligation, understood as a defeasible moral duty to obey the law, to one another. Those who break the laws fail to fulfill this obligation, and their failure justifies the law-abiding members, acting through the proper authorities, in punishing the lawbreakers. Rather than two separate problems, then, political obligation and punishment are two aspects of the same fundamental concern for sustaining a polity that its members can reasonably regard as a cooperative enterprise under the rule of law.