Endocrine therapy plays a pivotal role in the early treatment of estrogen receptor (ER)-positive breast cancer. Although evidence suggests that chemotherapy may work partly through ovarian ablation in young women who have ER-positive tumors, combined chemotherapy and endocrine therapy are generally advocated. In postmenopausal women, aromatase inhibition has become the new "gold standard" of treatment. More research is needed to define optimal regimens (aromatase inhibitor monotherapy versus tamoxifen sequential application), optimal duration of therapy and potential advantages of particular compounds. The optimal use of estrogen suppression (ovarian ablation with or without aromatase inhibition) and tamoxifen (administered sequentially or in concert with ovarian ablation) in premenopausal women has yet to be defined.