Sleep disorders and sleep of insufficient duration and quality have been associated with impaired cognitive functioning in typically developing children and in children with a wide array of disabilities and medical conditions. Among children with disabilities, those with intellectual disability, attention deficit hyperactivity disorder, and autism spectrum disorder have been found to have atypical patterns of sleep and high prevalence of sleep disorders. Following a description of how sleep is assessed, research relating sleep to cognitive functioning is reviewed for children with those three disabilities. Further needed research and implications for screening, diagnosis, prevention, and education are discussed.