Synthesized in 1966 and initially tested as a potential contraceptive, the nonsteroidal antiestrogen tamoxifen was subsequently found to have therapeutic benefits in the therapy of breast cancer. Today, tamoxifen is widely used in the treatment of this disease both in an adjuvant setting and for metastatic tumor. This article explores the history, pharmacology, and adverse effects of tamoxifen and summarizes the clinical evidence for its current indications and potential future use as a chemopreventative agent.