Evidence today indicates that the causes of mental retardation are biological, psychological, and social in origin and that a combination of these causes frequently occur in a single individual. Mental retardation is identified clinically by the presence of several signs that include, but are not limited to, a significant impairment of intelligence and a concurrent deficit in adaptive behavior. This monograph presents a report on the current state of knowledge about mental retardation. Research progress and objectives in the biomedical, behavioral, and social aspects of this handicap are discussed. Priority areas for future research are identified, including needs for biomedical research in biology, anatomy, chemistry, pharmacology, and physiology. A need is seen also for research in genetics and inborn errors of metabolism. Behavioral and social sciences research is called for in the areas of prevention of mental retardation, enhancing thinking and learning in retarded individuals, amelioration of the effects of mental retardation, and alternatives to institutionalization of retarded persons. A bibliography of 45 references is appended. (JD)