A woman’s hormonal milieu is critical in defining her risk of developing breast cancer. Over the last decade, much progress has been made in understanding the associations between circulating levels of endogenous hormones and breast cancer risk in postmenopausal women. Both estradiol and testosterone are associated with substantial increases in risk. Prolactin and insulin-like growth factor I also may predict later risk, although fewer data exist and findings are less consistent. Other hormones such as melatonin have only recently been evaluated in epidemiologic studies, but findings suggest a protective role in breast carcinogenesis. At this juncture, with several very robust hormone associations confirmed, an important question is how we can use this information to improve efforts in breast cancer prevention? In this talk, I will review our current understanding of the relationships between circulating hormones and breast cancer risk, and discuss important questions still to be answered in this arena.