This paper reviews the literature on gender differences in major depressive disorder (MDD) and bipolar disorder (BPD). Beginning in adolescence, women are at a higher risk than men of becoming depressed. Avenues of investigation that might ultimately help to explain this phenomenon include studies of gender differences in the processing of emotional stimuli, the psychotropic effects of gonadal steroids, and environment/gene interactions in men and women. With the exception of the elevated suicide rate among men, consistent gender differences in the course and symptoms of MDD have not been found. In BPD, women are more likely than men to develop a rapid-cycling course. Gender differences in treatment response, particularly in regard to mood stabilizing medications, warrant further study.