When an administrative agency’s interpretation of an ambiguous statutory term is challenged in court, the Chevron doctrine instructs judges to evaluate whether it is reasonable. But how does a court know reasonableness when it sees it? Here, I first show that reasonableness review is more complex than it might seem. Contrary to common images, for instance, courts do not determine a range of reasonable interpretations; and that is a good thing, because they are not competent to do so. Moreover, because traditional statutory interpretation approaches presume the existence of one correct meaning for a given word, they are not well suited to multivalent terms. But the Chevron doctrine assumes that multivalence permeates regulatory statutes. So ...