Insomnia is one of the most common and wide-spread sleep disorders. It includes difficulties of sleep initiation, sustaining and daytime impairment. A condition of cerebral hyperarousal plays the most important role in the genesis of insomnia. Cognitive, electrophysiological and metabolic parameters are correlated with hyperarousal state. According to the International classification of sleep disorders (ICSD-3), insomnia is divided into acute, chronic and unclassified. Treatment of insomnia includes specific and nonspecific approaches. Regardless of the origin of insomnia, sleep hygiene and behavioral therapy remain the methods of choice for the treatment.