The months following childbirth are a time of heightened vulnerability to depressive mood changes. Because of the abrupt and dramatic changes occurring in hormone levels after delivery, many studies have examined the role of hormonal factors in postpartum depression. The authors review the literature on potential hormonal etiologies in postpartum depression, in particular for progesterone, estrogen, prolactin, cortisol, oxytocin, thyroid, and vasopressin. While evidence for an etiologic role is lacking for most hormones, changes in certain hormonal axes may contribute to depressive mood changes in some women following childbirth.