For political realists, legitimacy is a central requirement for the desirability of political institutions. Their detractors contend that it is either descriptive, and thus devoid of critical potential, or it relies on some moralist value that realists reject. We defend a functionalist reading of realist legitimacy: descriptive legitimacy, that is, the capacity of a political institution to generate beliefs in its right to rule as opposed to commanding through coercion alone, is desirable in virtue of its functional role. First, descriptive (...) legitimacy plays an evaluative role: Institutions can fail to convince citizens that they have a right to rule and can be ranked by how well they do so. Second, descriptive legitimacy plays a normative role, because if an institution fails to convince subjects of its right to rule, this gives them a reason not to comply with its directives, even if it satisfies philosophers’ standards for possessing such right. (shrink)