To understand the postnatal development of the cerebral cortex we must know how changes in one cortical area depend on inputs from other cortical areas. Do cortical areas serving early stages of processing (primary sensory receiving areas) mature first, passing relatively stable outputs about sensorimotor relationships to cortical areas involved in higher stages of processing that are still developing? And, if some areas mature later than others, do they have functions that can account for aspects of adolescent behavior? Some observations support concurrent maturation in all cortical areas, others support a hierarchical sequence. Here, evidence on this important issue is evaluated, and means of obtaining reliable information are presented.