The current approach to mood disorders is that bipolar disorder, comprising both mania and depression, is a discreet illness distinct from unipolar depression. This formulation has profoundly influenced the approach to understanding the biology and etiology of these disorders, as well as the manner in which the various phases of bipolar disorder are treated. Our new model suggests that bipolar disorder comprises two distinct illnesses, mania and depression, and that bipolar depression is no different from unipolar depression. Studies of clinical syndromes, course of illness, family history and genetics, biological factors, and treatment response data directly or indirectly support this new model.