Recent developments in breast epithelial sampling techniques (nipple fluid aspiration, ductal lavage, and random fine needle aspiration) provide new opportunities for the acquisition of hormonal and cellular biomarker data in asymptomatic women, and thereby the possibility of developing a unified vision of how the hormonal environment of the breast may interact with the cellular expression of proteins, and with other evolving candidate markers of breast cancer risk. The purpose of this review is to integrate available information regarding cellular and breast fluid biomarkers of hormone action on the breast, to identify candidate biomarkers for studies of breast cancer risk and prevention. These include the estrogen receptors alpha and beta, markers of proliferative and apoptotic response, and protein markers of estrogen action in breast cells and nipple fluid. Studies of breast hormone levels in nipple aspiration fluid (NAF) show that estrone sulphate is present in large quantities in the normal breast, while the differences in serum ovarian steroids that are seen in pre- and postmenopausal women are blunted in NAF. The variability of several estradiol precursors in NAF over time is relatively small, a useful attribute of potential biomarkers of breast cancer risk, particularly if they are reversible with intervention in Phase 2 prevention trials. These studies are already providing new insights into the hormonal etiology of breast cancer, and should lead to the identification of robust, reversible biomarkers for use in breast cancer prevention studies.