To what extent can we as a community legitimately require individuals to contribute to producing public goods? Most of us think that, at least sometimes, refusing to pay for a public good that you have enjoyed can involve a kind of 'free riding' that makes it wrong. But what is less clear is under exactly which circumstances this is wrong. To work out the answer to that, we need to know why it is wrong. I argue that when free riding (...) is wrong, the reason is that it is unfair. That is not itself a very controversial claim. But spelling out why it is unfair allows us to see just which forms of free riding are wrong. Moreover, it supplies a basis from which some more controversial conclusions can be defended. Even if a public good is one that you have been given without asking for it or seeking it out, it can still be wrong not to be prepared to pay for it. It can be wrong not to be prepared to pay for public goods even when you do not receive them at all. And furthermore, it can be right to force you to do so. (shrink)