This article reviews the literature regarding endocrine factors postulated or presumed to be relevant in postpartum depression (PPD), a condition affecting at least 10% of childbearing women. The phenomenology and epidemiology of PPD are also described. Data suggest that parturition-related endocrine changes are causally implicated in PPD in a vulnerable subgroup of women. More specifically, studies by our group and others suggest a role for changes in estradiol and progesterone in precipitating mood symptoms among women with PPD. The mechanisms underlying such differential sensitivities remain undetermined. Future directions for research are explored.