Mental retardation is a syndrome that can be produced by many causative agents, acting alone or in combination. It involves about 5 million persons in the United States and in whom the affliction varies in symptomatology and severity. The fact that only 5% of the mentally retarded are in institutions emphasizes the contributions and responsibilities of the practicing physician. Research advances include new knowledge in the inborn errors of metabolism and chromosomal abnormalities. The implications of infections, prematurity, and perinatal events, the effects of some drugs, and the impact of certain psychologic forces are better understood. These recent findings have preventive and clinical implications. Other significant developments of importance in practice are the growth of community resources, new evidences of the capabilities of retardates, and increasing public awareness, exemplified by the appointment of President Kennedy's Panel on Mental Retardation.