In the treatment of breast cancer, especially in estrogen receptor (ER) positive patients, endocrine therapy has played an important role since bilateral oophorectomy, the first endocrine therapy, was performed by George Thomas Beaston in 1896. Thereafter, various therapeutic modalities such as radiation therapy to ovaries, surgical or medical adrenalectomy, or hypophysectomy, have been used for endocrine therapy in breast cancer. The discovery of ER and the development of anti-estrogens represent substantial progress, and tamoxifen, the first selective ER modulator, has become the gold standard in the endocrine therapy of breast cancer. The therapeutic effects of tamoxifen were confirmed by the 1995 Oxford overview. Recently, the 7th International Conference on Adjuvant Therapy of Primary Breast Cancer held in St. Gallen, Switzerland, recognized the increasing role played by endocrine therapy in properly selected patients groups, especially in younger patients with ER-positive tumors. In addition, recent advances in estrogen research and ER function at the molecular level have provided new strategies as well as a better understanding of endocrine therapy for breast cancer. Lately, new hormonal agents, such as the third-generation aromatase inhibitors, and ER downregulators, showed equivalent or better results in terms of therapeutic effects than tamoxifen. As a result, many clinical trials are ongoing to determine the most appropriate endocrine therapy for breast cancer. Therefore, it is important to maximize the benefits of endocrine therapy in clinical practice in terms of the patient's quality of life as well as the prolongation of patient survival. More studies are needed to determine optimal agents and the duration of therapy, combinations of agents or sequences of therapy according to prognostic and predictive factors.