The existence of bipolar disorder (BP) in youth is controversial. The current evidence regarding the diagnosis of BP in youth was reviewed. BP is a recurrent familial disorder that occurs in 1-3% of youth, particularly in adolescents. Except for subsyndromal BP, the prevalence of BP-I is similar across most countries. Due to the child's immaturity, the presence of comorbid disorders, and divergent interpretations of manic symptomatology it is difficult to diagnose BP in youth. Youth with subsyndromal mania and family history of BP, are at high risk to develop BP-I and BP-II. Both the full and subsyndromal syndromal BP are associated with significant psychosocial difficulties and increased risk for use of substances, suicidality, legal problems, and services utilization. BP disorder exists in youth, but it is difficult to diagnose. The recurrent nature and psychosocial morbidity associated with this illness during critical developmental stages calls for comprehensive longitudinal evaluation and accurate recognition and treatment because delays in treatment are associated with poor outcome.