The identification of the link between breast cancer and estrogens has led to the development of antiestrogens, in particular tamoxifen, to inhibit the activities of estrogen receptors (ERs) in breast cancer cells. The clinical use of tamoxifen has played a major part in decreasing breast cancer mortality over the past 30 years. Though antiestrogenic in the breast, some antiestrogens have estrogen-like actions in other tissues, acting to promote bone density and protect against cardiovascular disease, thus raising the possibility of their use in counteracting the effects of estrogen loss following menopause. Moreover, antiestrogens show efficacy as chemopreventive agents in women at high risk of developing breast cancer. Thus, antiestrogens define an important and well-understood class of cancer drug, which continue to be a mainstay in breast cancer treatment.