Publisher Summary Mental retardation is a categorization for a heterogeneous group of individuals with concurrent deficits in intellectual and adaptive functioning. Mental retardation is a deviation in development that may increase the risk of mental disorder and be accompanied by medical issues. The combination of developmental, medical, and psychiatric factors makes treatment complicated. The chapter discusses various intelligence tests used in the assessment and diagnosis of mental retardation. These tests include the Bayley Scales of Infant Development, Second Edition, the Bayley Scales of Infant and Toddler Development, Third Edition, the Differential Ability Scales (DAS), the McCarthy Scales of Children's Abilities, the Wechsler Preschool and Primary Scale of Intelligence (WPPSI-III), the Wechsler Intelligence Scale for Children-Revised (WISC-IV), the Wechsler Adult Intelligence Scale, Third Edition (WAIS-III), the Stanford-Binet Intelligence Scales, (SB5), the original Leiter International Performance Scale, and the Leiter International Performance Scale-Revised. Measures of intelligence play a crucial role in the assessment, diagnosis, and treatment of children and adolescents with mental retardation. Different tests emphasize various elements of the construct of intelligence. Awareness of the range of available tests allows using multiple assessment tools to derive the most accurate information regarding an individual's cognitive and adaptive ability. Adequate training, practice, and clinical supervision are the mainstays of competency standards for intelligence testing when confronted with the complex task of differentially diagnosing mental retardation from a host of similarly appearing developmental disorders and learning disabilities.