Mental retardation (MR) and other neurodevelopmental disabilities are problems encountered regularly in almost all pediatric clinical settings. This article reviews the detection, diagnosis, and management of children who have MR, including the role of the pediatrician as a supportive yet scientifically objective advisor to the parents who must learn gradually, as development unfolds, about the nature and severity of their child's handicap. The pediatrician also must be a resource to the family as they sort through the incomplete yet bewildering array of educational, therapeutic, and health services available to find what is most appropriate for their child. Definition MR was defined in 1983 by the American Association on Mental Deficiency as "significant subaverage general intellectual functioning existing concurrently with deficits in adaptive behavior and manifested during the developmental period." "Significant subaverage general intellectual functioning" is defined by an intelligence quotient (IQ) or developmental quotient (DQ) below 70 on formal psychometric testing. The psychological tests most commonly used in children are listed in Table 1. During the infant years, the Bayley Scales of Infant Development and the Cattell Infant Intelligence Scale are used; during the preschool years, the Stanford-Binet Intelligence Scale and the Wechsler Preschool and Primary Scale of Intelligence-Revised (WPPSI-R) are used; during school years, the Wechsler Intelligence Scale for Children-III (WISC-III) is used; and in adulthood, the Wechsler Adult Intelligence Scale (WAIS-R) is used.