Insomnia is a common problem in the general population and has a higher prevalence in persons with medical and psychiatric disorders. Although insomnia is most often transient, occurring as a result of identifiable stressors, a substantial portion of insomnia cases involve persistent sleep difficulty. This chronic form of insomnia may be associated with a wide range of adverse consequences. An understanding of the characteristics and causes of this disorder and the available therapeutic strategies will promote more effective identification and treatment of patients with chronic insomnia.