The choice of endocrine agent for breast cancer depends on the menopausal status of the patient, the stage of disease, prognostic factors, and the toxicity profile of the agent. Endocrine therapies are typically given sequentially, with the least toxic therapy given first. Tamoxifen is considered first-line endocrine therapy for all stages of breast cancer. New antiestrogens in development include nonsteroidal agents related to tamoxifen and pure steroidal antiestrogens. Luteinizing hormone-releasing hormone agonists are an effective form of endocrine therapy for premenopausal women with advanced breast cancer, and aromatase inhibitors are effective in postmenopausal women. Newer and more selective aromatase inhibitors that are p.o. active and have improved side-effect profiles have been developed. Recent trials have found these agents to improve survival in comparison to the progestins; thus, aromatase inhibitors are replacing progestins as second-line therapy for metastatic disease. Current trials are examining the potential role of aromatase inhibitors as first-line therapy for metastatic disease or as adjuvant therapy for early disease. The antiprogestins and antiandrogens studied thus far have had only limited success in breast cancer clinical trials.