The 2017 International Society for Bipolar Disorders (ISBD) Task Force report on pediatric bipolar disorder (BD) cemented the concept that BD can have onset before the age 18, and that establishing the diagnosis should be based on symptoms of mania or hypomania, while chronic irritability by itself is not sufficient to establish a BD diagnosis [1, 2]. A 2010 study of 1566 patients from six international sites documented that approximately 5% of BD-1 and 5% of BD-2 cases had onset before the age of 20 (Figure 7.1), although the authors noted that only 34.1% of patients were evaluated at onset of their BD, so investigators had to rely on patient recall for two-thirds of the sample.