Insomnia is a common and clinically important problem. It may arise directly from a sleep-wake regulatory dysfunction and/or indirectly result from comorbid psychiatric, behavioral, medical, or neurological conditions. As an important public-health problem, insomnia requires accurate diagnosis and effective treatment. Insomnia is primarily diagnosed clinically with a detailed medical, psychiatric, and sleep history. Polysomnography is indicated when a sleep-related breathing disorder or periodic limb movement disorder is suspected, initial diagnosis is uncertain, treatment fails, or precipitous arousals occur with violent or injurious behavior. However, polysomnography is not indicated for the routine evaluation of transient insomnia, chronic insomnia, or insomnia associated with psychiatric disorders.