Adjuvant endocrine therapy with the selective estrogen receptor modulator, tamoxifen, has significantly improved mortality from early-stage breast cancer for both pre- and postmenopausal women with hormone receptor-positive breast cancer. Recent large clinical trials have demonstrated a clear and consistent benefit for the incorporation of aromatase inhibitor (AI) therapy within adjuvant endocrine regimens for postmenopausal women. The AIs, which are associated with myalgias, arthralgias, and a reduction in bone mineral density, are generally well tolerated and have lower risks of endometrial carcinoma and thromboembolic events than tamoxifen. Data are awaited from ongoing trials to better define the optimal sequencing strategy, duration, and AI agent. Attempts to further optimize adjuvant endocrine therapy by identifying predictive biomarkers of response, as well as by developing strategies to overcome endocrine resistance are underway. In premenopausal women AI monotherapy is currently contraindicated and tamoxifen remains the standard of care. The role of ovarian function suppression in addition to tamoxifen or combined with AI therapy is being explored. The hope is that continued advances in endocrine therapy will translate into improved survival among both pre- and postmenopausal women with receptor-positive breast cancer.