Abstract: Approximately three-quarters of patients with invasive breast cancer present with hormone receptor-positive disease, and modulation of estrogen receptor(ER) activation is an essential component of systemic adjuvant therapy for these women. As the estrogen receptor (ER) pathway is key to cancer cell growth, modulation of ER activation is crucial. Tamoxifen, a selective estrogen receptor modulator, has traditionally been the mainstay of adjuvant endocrine therapy in pre- and postmenopausal women. Recent trials evaluating the use of aromatase inhibitors (AIs) have challenged this standard procedure in postmenopausal women. This paper reviews the progress in endocrine therapy for postmenopausal women with breast cancer.