This chapter includes disorders subsumed under the broad category of sleep disorders in DSM-IV-TR (American Psychiatric Association 2000). The category is organized into four sections. The first section, that of the dyssomnias, includes disorders of the amount, quality, or timing of sleep, such as primary insomnia, primary hypersomnia, narcolepsy, breathing-related sleep disorder, dyssomnia not otherwise specified, and circadian rhythm sleep disorder. Treatment principles for the excessive sleepiness of primary hypersomnia are similar to those for narcolepsy; therefore, primary hypersomnia will not be discussed separately in this chapter. The second section is comprised of the parasomnias, disorders involving abnormal behavioral or physiological events occurring in association with sleep, specific sleep stages, or sleep–wake transitions. The final two DSM-IV-TR sleep disorder sections pertain to sleep disorders related to another mental disorder and to a general medical condition, respectively. Disorders falling within these latter two sections are not addressed in our review, because they comprise a heterogeneous group of disorders involving sleep-related symptoms secondary to other conditions that are more thoroughly reviewed in other sections of this volume. This chapter covers disorders encountered during both childhood and adulthood.