Theories of social justice propose various principles for the just regulation of social institutions and practices. However, why should individuals comply with the rules of just social institutions? To answer this question, a theory of obligation is required. This thesis examines and defends the principle of fairness as a theory of individual obligation. It begins by reviewing the debates within political philosophy over the principle since its initial formulation by H. L. A. Hart. Thereafter, the defence of the principle of fairness proceeds in three stages. First, the thesis explores the moral foundations of the principle. It is argued that the concepts of right, equality, and fairness as reciprocity work as the moral foundations provided that fairness as reciprocity has priority over right and equality. On the basis of this reciprocity-based foundation, a revised principle of fairness is stated, which specifies the conditions for the justification of an obligation to follow institutional rules. Second, the thesis rebuts main objections levelled against the principle: the consent argument, the limiting argument, and the utility argument. Finally, the principle of fairness is defended as a principle that has appealing normative implications for one of the most important challenges we face in recent times, global climate change. With theoretical elaboration, defence against main objections, and practical application, this thesis presents a comprehensive development of the principle of fairness as a plausible theory of obligation.