In this essay, I argue that the only account of legal obligation that can succeed is one that holds that legal obligations, by nature, bind by providing prudential reasons (i.e. reasons that appeal to one’s self-interest). Then I consider a number of potentially conflicting views. First, I consider the possibility that mandatory legal norms merely “purport” to define legal obligations and hence reasons for action. On this view, mandatory legal norms do not necessarily give rise to legal obligations; they merely “purport” to do so; I reject this view on the ground that it is inconsistent with the core legal practices that construct the content of our concept of legal obligation. Second, I consider a volunteerist theory of legal obligation, according to which whether or not a putative law subject is legally obligated by a mandatory legal norm depends on whether the subject has, in some sense, “chosen” to be obligated. On such a view, mandatory legal norms neither necessarily define legal obligations nor provide reasons for action; they simply purport to do both, leaving open the possibility of a legal system consisting of laws that never provide reasons for action. I argue that this view cannot, among other problems, explain how it is legal obligations bind subjects in the sense of making certain acts non-optional in the relevant sense.Finally, I consider a view of legal normativity that holds that legal obligations, as a conceptual matter, give rise to moral reasons for action. I argue, in response, that there are, as many normative political theorists have noted, many mandatory norms properly characterized as “law” that are not plausibly thought of as giving rise to even a prima facie moral reason for action.