This issue of Endocrinology Clinics brings the reader up to date on the latest information about hormones and cancer of the breast and prostate. The first section focuses on the breast, and topics covered include the following. The role of sex steroids and their receptors in normal breast development; estrogen carcinogenesis in breast cancer; hormonal mechanisms underlying the relationship between obesity and breast cancer; postmenopausal hormone replacement therapy and the risk of breast cancer; aromatase inhibitors, anti-estrogen and SERMS in the treatment of breast cancer; and androgens in breast cancer in men and women. The second section is devoted to the prostate, and topics covered include the following. Overview of prostate anatomy, histology, and pathology; the critical role of sex steroids in normal prostate development; estrogens and androgens in prostate cancer development and the rationale for hormonal chemopreventive therapies; weighing the clinical evidence regarding the timing and extent of androgen ablative therapy for prostate cancer treatment; new hormonal therapies for castration-resistant prostate cancer; and the management of the side effects of castration therapy.