Bipolar disorder is one of the most frequent and severe mental disorders. A generally recurrent or chronic course is often predominated by bipolar depression. Genetic, neurobiological, and psychosocial factors are supposed to be relevant in the pathophysiology of this disorder. Current data have implicated dysfunction of neurotransmitter, neuroendocrine, and intracellular signaling networks. Neuroanatomic changes in bipolar disorder have been reported, too. In this article, we describe current knowledge regarding epidemiology, etiopathogenesis, and course of bipolar disorder with a focus on depression of bipolar disorder.