In reviewing recent findings on affective conditions in the interface of unipolar and bipolar disorders, we find evidence favoring a partial return to Kraepelin's broad concept of manic-depressive illness, which included many recurrent depressives and temperamental variants. This review addresses methodologic, clinical, and familial considerations in the definition and characterization of a proposed spectrum of bipolar disorders which subsumes episodic and chronic forms. Episodic bipolar disorders are subclassified into bipolar schizoaffective, and bipolar I and II, and bipolar III or pseudo-unipolar forms. Chronic bipolar disorders could be either intermittent or persistent, and are subclassified into chronic mania, protracted mixed states, and rapid-cycling forms, as well as the classical temperaments (cyclothymic, hyperthymic, irritable and dysthymic).