The neurobiological basis of bipolar affective disorders is unknown. However, neuroanatomic circuits of mood regulation have been hypothesized. Neuroimaging revealed volumetric changes of specific brain structures in these circuits. The most prominent abnormality is enlargement of the amygdala. In addition there might be structural changes in the frontal lobe, cerebellum, and pituitary. The findings in bipolar disorder differ from those in unipolar depression and schizophrenia. For further identification of the neurobiological basis of bipolar disorders, structural neuroimaging combined with functional neuroimaging such as magnetic resonance spectroscopy, neuroendocrinological studies, and genetical analyses are required to subgroup patients with bipolar disorder by diagnostic, prognostic, and therapeutic criteria.