The frequent occurrence of depressive symptoms in patients with borderline personality disorder has generated considerable interest in the nature of the relationship between borderline personality disorder and the depressive disorders. Data from the perspectives of phenomenology, biology, family history, course of illness, comorbidity patterns, and treatment response have been brought to bear on the question. Reviews based on research available by 1985 and 1991, respectively, arrived at differing conclusions: (1) that both disorders shared common but non-specific sources, and (2) that the two disorders were unrelated but co-occurred because of the high prevalence of each. Since the time of these reviews, additional evidence has become available from a wider range of biological investigations, better controlled comorbidity studies, studies of the relationship of psychosocial stressors to the course of each disorder and neuroimaging studies. In reviewing the more recent findings, we propose the less parsimonious hypothesis that the disorders co-occur, both because they share some common biological features and because the psychosocial sequella of each can contribute to the development of the other.