Intellectual disability (previously termed mental retardation) is a developmental disorder with onset prior to age 18 years characterized by impairments in measured intellectual performance and adaptive skills across multiple domains (Table 21–2). DSM-IV-TR classifies mental retardation as an Axis II disorder, segregated from most primary psychiatric disorders. The separation of developmental disorders from mental disorders is an attempt to establish and maintain the boundary between these conditions, and to remind clinicians that intellectual disability is not synonymous with psychiatric illness (Harris 2006).