Insomnia is a prevalent public health problem affecting large segments of the population on an occasional, recurrent, or chronic basis. Persistent insomnia is associated with significant impairments in daytime functioning, reduced quality of life, and increased health care costs. This chapter reviews several theories of insomnia and sleep, including Borbely's two process theory, the three‐P model (which emphasizes predisposing, precipitating, and perpetuating factors), the behavioral model, the cognitive model, hyperarousal models, the neurocognitive model, several hybrid models, and the role of environmental factors. Diagnosis, assessment, and case formulation are discussed, with a focus on development (children and teens have a different sleep need relative to adults) and the presence of comorbidity. Cognitive behavioral therapy for insomnia is highlighted. Cognitive behavioral therapy for insomnia is an effective and well‐accepted treatment for insomnia. The pros and cons of taking a cognitive behavioral therapy approach to the treatment of insomnia are contrasted with a pharmacologic approach to treatment. Finally, a brief overview of other sleep disorders is included; namely, circadian rhythm disorders, hypersomnia, sleep apnea, narcolepsy, restless legs syndrome, and periodic limb movement disorder. Exogenous factors such as social rhythms, which may have a powerful impact on sleep, are also highlighted.