Intellectual disabilities are disabilities characterized by significant limitations both in intellectual functioning and in adaptive behavior, which covers many everyday social and practical skills. Approximately one to three percent of school-aged children suffer with an intellectual disability. Most are classified as familial intellectual disabilities in which there is no apparent cause, but there is a history of impairment within the family. The most common biological causes are fetal alcohol syndrome and down syndrome.

Ninety percent of those with an intellectual disability, are classified as having a mild intellectual disability with IQ scores ranging between 50-70. Some may not be diagnosed prior to starting school. Earlier development is often slower than average. Those with a mild intellectual disability can read the third to sixth grade in school and can go on to hold jobs and function independently.

Five to ten percent are classified as having moderate intellectual disabilities in which IQ scores range from 35-50. They tend to be slow to develop language and motor skills, generally cannot progress beyond the second grade and are capable of occupational training and social skills, but typically need supervision.

A small number are classified as having severe intellectual disabilities in which IQ scores range from 20-30 or profoundly disabled, with IQ scores less than 20. Among those with severe or profound disabilities there is often no speech, poor motor control, and the need for 24-hour care. Some may learn basic self-care tasks and may be partially independent as adults, but most are institutionalized for the majority of their lives.

Laws call for the education of all handicapped children in the least restrictive environment possible. The intent is to provide children with disabilities an education in a setting most similar to those of children without special needs. This practice came to be known as mainstreaming. Through mainstreaming children with disabilities were integrated as much as possible into the traditional educational system and provided with a range of educational alternative. Some promote full inclusion or the integration of all students, even those with the most severe disabilities into regular classes and all other aspects of school and community life.