This chapter presents an overview of the historical aspects of psychosis in intellectual disabilities. Additionally, the current knowledge about prevalence, age of onset, symptoms, and prognosis. Aetiology is included in the chapter about biological aspects. From the late 1800s, severe mental health conditions and “mental retardation” (now intellectual disabilities) were seen as closely connected, which is reflected in the International Classification of Diseases, ICD, in which both mental health conditions and intellectual disabilities, including neurodevelopmental disorders, up until the last decades have been placed in the same chapter. In this book, the current diagnostic subgroups of psychosis will not be presented or discussed. Psychosis in bipolar disorder and major depression will not be presented or discussed. This book emphasizes phenomenology, diagnostics, biological aspects, interventions and treatment, and user’ and their family’s experiences.