Endocrine therapy, targeting estrogen production or the estrogen receptor, is a crucial component in the treatment of breast cancer. This has been recognized for over a century. Ovarian ablation is the oldest form of endocrine therapy, first used in 1896. This presentation will review: the history of endocrine therapy in the treatment of breast cancer; endocrine strategies in premenopausal and postmenopausal women; data on the use of endocrine therapy in early breast cancer; new directions in the endocrine therapy of metastatic breast cancer; and clinical implications of HER2-neu status and endocrine therapy.