This chapter reviews the assessment and differential diagnosis of sleep disorders in the elderly. Cognitive deficits and sleep difficulties are normal age-related changes, may explain an elderly patient's complaints about sleep. There are also a variety of other diagnoses that must be considered, including the presence of specific sleep disorders, circadian rhythm disturbances, and medical and psychiatric co-morbidities. There are also several less common presentations of sleep disordered breathing (SDB) that can be found in the elderly, which include complaints of insomnia, nocturnal confusion, and daytime cognitive impairment such as short-term memory loss and poor concentration. As with younger patients, sleep complaints in older patients should be evaluated with a thorough sleep history and often an overnight sleep study. The appropriate diagnosis or identification of specific, often reversible or correctable factors contributing to an elderly patient's sleep complaint can result in significant improvements in quality of life and daytime functioning.