Circadian rhythm sleep disorders are characterized by a misalignment between the timing of sleep–wake and the external environment that results in insomnia, excessive daytime sleepiness, and impaired waking function. This chapter covers sleep disorders thought to be result of alterations in the endogenous circadian clock, such as delayed sleep phase type, advanced sleep phase type, free-running type, and irregular sleep–wake type, and those that result in circadian misalignment as a result of alterations in the external environment, such as shiftwork type and jetlag type. The clinical presentation, prevalence, pathophysiology, and treatment options are discussed. Comprehensive treatment should take into account behavioral and environmental factors that influence sleep–wake and is aimed at realignment of the sleep–wake cycle with desired times using circadian synchronizing agents such as light and melatonin in combination with good sleep hygiene.