Ideally authentic performances of musical works execute the work-identifying prescriptions issued by the composer, usually in scores. The performer must understand the conventions for recovering the instructions from the notation and be able to perform on appropriate instruments. I reject an alternative that sees authenticity as aiming to recreate under modern conditions aesthetic effects originally intended by the composer. Authenticity is a performance value because is important to delivering the target work, but its value is not overriding. Some authenticity can be sacrificed for practical reasons, but only rarely is deliberate inauthenticity justified for the sake of interpretational variety.