An intellectual disability is defined as significantly below-average general mental ability that occurs during the developmental period from conception through age 18 that coexists with problems in adaptation and significantly influences a child’s experience and performance in preschool, kindergarten, elementary school, and beyond. Specifically, children with intellectual disabilities are determined at any point between conception through 21 to have low general intelligence that is significantly different from that of peers, and to demonstrate problems with adaptation to the environment, especially school and educational experiences. This entry provides information about the causes of intellectual disabilities, characteristics, diagnosis, issues related to early childhood education from birth through age 8, and education.