Considerable data implicate estrogens in breast cancer carcinogenesis and progression. In the postmenopausal woman, estrogens are produced in breast tissues and many other sites throughout the body when androgen precursors are converted into estrogens via the enzyme aromatase. Inhibition of this enzyme with aromatase inhibitors (AIs) has demonstrated reductions in systemic as well as intratumoral estrogens. These drugs have now been utilized in large phase 3 randomized trials and have led to greater improved clinical benefit than the "gold standard," tamoxifen. Questions remain about the long-term side effects and safety profile of AIs. They are associated with increasing incidence of osteoporosis and bone fractures. Nevertheless, AIs add to our armamentarium for therapy and possible prevention of breast cancer.