Kraepelin had envisaged a broad concept of manio-depressive illness that included recurrent depressions. The unipolar-bipolar dichotomy restricted the territory of manic-depression to strictly defined bipolar disorder with mania (bipolar I). Research over the past three decades has shown that bipolarity extends into the severe psychotic domain, as well as into the interface between bipolarity and unipolarity. At the severe end of the spectrum, familial-genetic and course parameters support the extension of bipolar disorder into "schizo-bipolar." At the "softer end," bipolar II, III, and IV have been described. The latter are distinguished from bipolar [by excited periods that are non-psychotic and brief; and sometimes adaptive (hypomania as short as two days), could be occasioned by antidepressants, or constitute temperamental characteristics along cyclothymic and hyperthymic lines. The clinical and familial data in support for extending the bipolar spectrum have come from U.S. and European centers and community studies. The broadened clinical spectrum does not necessarily imply genetic homogeneiry; indirect evidence supports underlying polygenic or oligogenic inheritance. Finally, the broad spectrum has important therapeutic and public health significance in terms of early intervention and extending the benefit of mood stabilizers to conditions that might otherwise be diagnosed“unipolar” or “impulse control disorders.”