Children and adolescents with bipolar disorder (BP) usually have poor psychosocial outcome, increased risk for suicide, substance abuse, and psychosis, indicating the need for accurate diagnosis and prompt treatment of this illness. In clinical samples the incidence of BP disorder in children and adolescents has ranged from 2% to 15% depending on the nomenclature system, assessment instruments, and methodology used to diagnose patients' psychiatric disorders and sample origin. Patients with Kraepelinian or diagnostic and statistical manual of mental disorders (DSM-IV) classical BP disorder display the cognitive, emotional, and behavioral BP symptomatology described in adult BP populations, with a great proportion having mixed and rapid cycles. Early-onset BP disorder has severe negative psychosocial and academic consequences that can be potentially ameliorated by proper diagnosis and treatment. Patients with mixed presentations and/or rapid cycles appear to have a poorer response to the mood stabilizers.