The possibility that infants' and young children's immature behaviors and cognitions are sometimes adaptive is explored and interpreted in terms of evolutionary theory. It is argued that developmental immaturity had an adaptive role in evolution and continues to have an adaptive role in human development. The role of developmental retardation in human evolution is discussed, followed by an examination of the relation between humans' extended childhood and brain plasticity. Behavioral neoteny, as exemplified by play, is examined, as are some potentially adaptive aspects of infants' perception and cognition that limit the amount of information they can process. Aspects of immature cognition during early childhood that may have some contemporaneous adaptive value are also discussed. It is proposed that viewing immaturity as sometimes adaptive to the developing child alters how children and their development are viewed.