Insomnia is a common problem that is chronic for many individuals. Multiple processes, including psychologic, physiologic, and environmental factors, can influence insomnia, and their effects can shift over time. Accordingly, the evaluation of patients who have insomnia should be broad in scope. Insomnia represents a clinical problem with significant daytime consequences, associations, and comorbidities. Several nosologies categorize insomnia into specific diagnoses. Useful minimal criteria for an insomnia disorder include persistent nighttime symptoms with daytime distress or impairment. Specific treatments, addressing particular underlying causes,and general pharmacologic and nonpharmacologic strategies may play valuable roles in the management of insomnia patients. The effective treatment of insomnia can have further benefits in decreasing the likelihood of future psychiatric symptoms.