In up to 75% of breast cancers, estrogen receptor (ER) signaling is a key promoter of tumor proliferation, and inhibition of this pathway has clear therapeutic efficacy. The principal clinical means of inhibiting ER signaling comprise selective ER modulators, such as tamoxifen, that act as partial receptor agonists; measures to reduce the circulating level of estrogen, including ovarian ablation, gonadotropin-releasing hormone analogues, and aromatase inhibition; and antagonism and downregulation of ER by the antiestrogen fulvestrant. Each of these therapies is effective in a proportion of ER-positive breast cancers, but de novo and acquired resistance remain significant problems. Emerging knowledge of the biology of ER signaling will provide insights into the mechanisms of resistance and help guide development of therapeutic strategies to maximize response. This review summarizes the contemporary treatment of early-stage and advanced ER-positive breast cancer in premenopausal and postmenopausal women, with an emphasis on recently published or presented data. Mechanisms of resistance to endocrine interventions and trials exploring strategies to overcome them will also be discussed.