The auditory system of many mammalian species is immature at the time of birth. Peripheral elements, particularly the middle ear and cochlea, account for much of the physiological immaturity observed in central auditory structures. However, there is now considerable evidence that the central pathway undergoes developmental changes that, at least partially, occur in parallel with peripheral development. Auditory nerve fibres and their terminals have been shown to be of smaller diameter and less extensively arborized in neonatal than in adult cats. These factors almost certainly contribute to many of the sluggish physiological properties of neurones in the kitten auditory nerve and cochlear nucleus. At higher levels of the cat auditory system (inferior colliculus, auditory cortex) mechanisms subserving binaural interaction also undergo a period of postnatal development. Recent studies of sound deprivation produced by either deafferentation or a conductive hearing loss have demonstrated that alteration of cochlear output during the immediate postnatal period may change the normal development of the central auditory system.