Smith–Magenis syndrome (SMS) is a rare microdeletion syndrome of chromosome 17p11.2 associated with a specific pattern of physical, developmental, and behavioral characteristics. The estimate of incidence prior to the early 2000s was approximately 1 in 25,000 individuals; however, this has changed over time to approximately 1 in 15,000 individuals. The neurobehavioral phenotype is distinct and complex, characterized by high frequency of outbursts and tantrums, attention seeking, impulsivity, aggression, hyperactivity, distractibility, toileting difficulties, stereotypies (repetitive or self-stimulatory behaviors), sleep disturbance, and self-injurious behaviors. This unique constellation of physical, psychological (cognitive and behavioral), and social abnormalities both directly and indirectly affects development. This entry examines the genetics and epidemiology of SMS, as well as its psychological comorbidities.