Four studies examined preschoolers' sensitivity to agents' knowledge of conventional forms. Three- to 4-year-olds heard a speaker apply either conventional or wrong labels to familiar objects (Studies 1 and 2, N = 57) or peculiar but correct labels (Study 3, N = 19). When then asked by the speaker for the referent of a novel label, children exposed to an accurate labeler were more likely to choose an unfamiliar object than children exposed to an inaccurate labeler. Study 4 (N = 36) replicated these findings using object functions instead of labels. Children hold an assumption of conventionality with regard to both object labels and functions, but they are selective in their application of this assumption toward agents who are knowledgeable of conventions in these domains.