This paper presents a moral philosophical account of free riding, specifying the conditions under which failing to pay for nonrival goods is unfair. These conditions do not include the voluntary acceptance of the goods: this controversial claim is supported on the strength of a characterization of the kind of unfairness displayed in paradigm cases of free riding. Thus a "Principle of Fairness" can potentially serve as a foundation for political obligations. The paper also discusses the relation between its moral philosophical (...) account of free riding, and a game theoretic or economic account. (shrink)