Subthreshold bipolar disorder (BP) is a common, understudied, illness associated with high levels of impairment. "Subthreshold" BP refers to individuals who have episodes of both depression and hypomania, but the episodes of hypomania do not last long enough or are characterized by too few symptoms to meet conventional criteria for "threshold" hypomania. Compared to individuals with episodes of depression only (major depressive disorder or MDD), individuals with subthreshold BP have higher rates of suicide, earlier onset of illness, more episodes of depression, and more co-occurring psychiatric disorders. Despite the severity of the disorder, virtually nothing is known about how to treat this illness.