An inability to sleep or sleep prematurely ended or interrupted by periods of wakefulness (insomnia) are some of the most frequent complaints heard from patients. Insomnia can be situationally related or persistent in nature. Persistent insomnia may be associated with biological rhythm disturbances, drug dependency, psychophysiologic abnormalities, psychiatric disturbance, sleep apnea syndrome or nocturnal myoclonus. This article describes these pathologies, gives clues toward differential diagnosis, suggests patient subgroups that would benefit from referral for specialized evaluation at a sleep disorder center and describes current treatment options.