The meritocratic principle of educational justice maintains that it is unfair that individuals with similar ability who invest equal effort, have unequal educational prospects. In this paper I argue that the conception of ability that meritocracy assumes, namely as an innate trait, is critically flawed. Absent a coherent conception of ability, meritocracy loses its ability to morally evaluate educational practices and policies, rendering it an unworkable principle of educational justice. Replacing innate ability with an alternative conception of ability is, therefore, (...) crucial for meritocratic educational justice. I propose incorporating an alternative conception of ability into meritocracy—as the ″current limits of student ability″. The account of meritocracy that follows entails that unequal educational prospects are fair only when they result from the constraints of individual potential (or from differential effort). I argue that this potential-based account of meritocracy, though demanding, is a plausible and attractive account of educational justice. (shrink)