The longitudinal course of children and adolescents with bipolar disorder (BP) is manifested by frequent changes in symptom polarity with a fluctuating course showing a dimensional continuum of bipolar symptom severity from subsyndromal to mood syndromes meeting full Diagnostic and Statistical Manual of Mental Disorders criteria. These rapid fluctuations in mood appear to be more accentuated than in adults with BP, and combined with the high rate of comorbid disorders and the child's cognitive and emotional developmental stage, may explain the difficulties encountered diagnosing and treating BP youth. Children and adolescents with early-onset, low socioeconomic status, subsyndromal mood symptoms, long duration of illness, rapid mood fluctuation, mixed presentations, psychosis, comorbid disorders, and family psychopathology appear to have worse longitudinal outcome. BP in children and adolescents is associated with high rates of hospitalizations, psychosis, suicidal behaviors, substance abuse, family and legal problems, as well as poor psychosocial functioning. These factors, in addition to the enduring and rapid changeability of symptoms of this illness from very early in life, and at crucial stages in their lives, deprive BP children of the opportunity for normal psychosocial development. Thus, early recognition and treatment of BP in children and adolescents is of utmost importance.