Anatomical investigations were carried out on 39 human brains ranging in age from the 29th week of development to the 70th year of life. It was demonstrated that myelination, as a prerequisite for functional maturation, takes place during the 1st year of life. Functional maturation was analyzed with electrophysiological techniques. Normal development depends on adequate stimulation during certain sensitive periods in the 1st year of life as well as the continuous use of the central auditory and speech-hearing systems.