Intellectual disability (identified as mental retardation in the International Classification of Diseases, 10th revision [ICD-10]) has been a marginal area for health care and health research. In most countries, it receives little or no attention during medical training, and a large divide exists between availability of services and the health needs of affected individuals. A research gap is also present between intellectual disability and other neuropsychiatric disorders, which largely contributes to the invisibility of the disorder in global health policy. Paradoxically, the prevalence of intellectual disability, its associated burden, and related costs are all high. Conservative estimates suggest a prevalence of 0·7–8·0%, with larger differences for mild versus severe intellectual disability. Furthermore, in low-income countries, the prevalence of severe intellectual disability is more than 0·5% (at least double that of high-income countries). This excess is related to preventable causes, such as malnutrition, iodine deficiency, birth trauma, or lead poisoning. All these facts make intellectual disability a public health issue, whereby closing the gap between needs and resources should become a priority, as shown clearly in WHO's Atlas on intellectual disability.