Variability in the classification of depressive illness greatly handicapped the work of geneticists prior to the application of the polarity concept. The past decade has however, seen a burgeoning of interest in this area, with researchers concentrating on bipolar illness, this being the most clearly defined entity. Findings of a lifetime morbidity risk for affective illness of 15-20% in the first degree relatives of bipolar patients has been usual. This compares with 10-15% for patients with unipolar illness. Transmission patterns indicative of known forms of Mendelian inheritance have not been consistently demonstrated. Genetic markers including colour blindness and HLA antigens have been pursued avidly but have yielded inconclusive results. Further progress seems unlikely unless more sub-groups are defined using biochemical and physiological markers.