Data are reviewed on the descriptive epidemiology of commonly occurring DSM-IV mental disorders in the United States. These disorders are highly prevalent: Roughly half the population meets criteria for one or more such disorders in their lifetimes, and roughly one fourth of the population meets criteria in any given year. Most people with a history of mental disorder had first onsets in childhood or adolescence. Later onsets typically involve comorbid disorders. Some anxiety disorders (phobias, separation anxiety disorder) and impulse-control disorders have the earliest age of onset distributions. Other anxiety disorders (panic disorder, generalized anxiety disorder, post-traumatic stress disorder), mood disorders, and substance disorders typically have later ages of onset. Given that most seriously impairing and persistent adult mental disorders are associated with child-adolescent onsets and high comorbidity, increased efforts are needed to study the public health implications of early detection and treatment of initially mild and currently largely untreated child-adolescent disorders.