Experts play an important role in society, but there has been little investigation about the nature of expertise. I argue that there are two kinds of experts: those whose expertise is a function of what they know (epistemic expertise), or what they do (performative expertise). Epistemic expertise is the capacity to provide strong justifications for a range of propositions in a domain, while performative expertise is the capacity to perform a skill well according to the rules and virtues of a practice. Both epistemic and performative experts may legitimately disagree with one another, and the two senses are conceptually and logically distinct.