This article explores the important relationship between the neurodevelopmental, neuropsychiatric condition attention deficit hyperactivity disorder (ADHD) and intellectual disability. ADHD can be associated with a range of specific learning difficulties. In this article, we focus on the global, substantial and enduring cognitive delays, usually of early onset, as defined in the International Statistical Classification of Diseases and Related Health problems, 10th Revision (ICD-10) and the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, Text Revision (DSM-IV-TR) classifications of mental retardation.