The Principle of Fairness and Natural Duties defends the Principle of Fairness (PoF) as a widely applicable moral principle that can ground obligations to obey the law. The first three chapters develop an interpretation of the PoF by responding to criticisms from A. John Simmons. Simmons argues that, for the principle of fairness to generate obligations, beneficiaries of cooperative public-goods schemes must voluntarily accept the benefits they receive. I argue against Simmons's voluntarism by showing that, even on his view, mere receipt of benefits that are worth their cost can trigger fairness obligations. In a similar vein, Simmons argues that the PoF requires a type of cooperation that we do not see in political communities. Thus, even if his voluntarism argument fails, Simmons can conclude that the PoF cannot ground political obligation. In response, I argue that the cooperation condition can be removed from the PoF entirely. Simmons's arguments defend the broader claim that the PoF applies to few individuals and generates few obligations. In pushing back against this conclusion, I argue that the PoF applies not only to natural agents, such as humans, but also to some artificial agents such as governments and corporations. While the PoF is a robust moral principle, it may not be able to satisfactorily ground political obligation. The last two chapters of this dissertation, therefore, explore the possibility of combining the PoF with natural duties. This combination has been attempted several times in the literature on political obligation. Most often, the PoF is combined with some version of the natural duty to rescue. I argue that this duty can be combined with the PoF, but that the combination cannot ground political obligation. Rather, they ground an obligation to perform one's fair share of global rescues, though each individual has some discretion in choosing which rescue efforts to contribute to. To ground political obligation, the PoF must be combined with the natural duty of justice. The two principles are mutually reinforcing, answering objections that each, alone, cannot overcome. This combination leads to a novel theory of political obligation.