Intellectual disability (ID), formerly known as mental retardation, is characterized as a disorder marked by impairments in both intellectual and adaptive functioning. Furthermore, the onset of the disability must occur during the developmental period (i.e., prior to the age of 18 years). The diagnosis of ID is rare, with approximately 2% to 3% of the general population meeting diagnostic criteria. This entry focuses on the characteristics associated with ID at a moderate level of severity. Specifically, this entry defines ID as several impairments that may be present in individuals who fall within this category.