The way children evaluate the reporting of peers' transgressions to authority figures was investigated. Participants, ages 6-11 years (N = 60), were presented with a series of vignettes, each of which depicted a child who committed either a minor transgression (such as not finishing the vegetables at lunch) or a more serious transgression (such as stealing from a classmate). Participants were asked to evaluate the decision of a child observer who either did or did not report the transgression to a teacher. Younger children considered reporting to be appropriate for both types of transgressions, but older children considered reporting to be appropriate for major transgressions only. Results are interpreted with reference to (a) a changing peer culture in which the social cost of reporting transgressions increases and (b) a developmental change in children's cognitive capabilities. (Contains 2 tables and 2 figures.)