Over several decades, investigators working through National Cancer Institute-sponsored Cooperative Groups have contributed to major advances in the endocrine treatment of breast cancer. Accomplishments include the benefit of tamoxifen therapy for early stage invasive and noninvasive breast cancer, the benefit of raloxifene and tamoxifen for prevention of breast cancer, the improved efficacy of tamoxifen after chemotherapy as opposed to concurrent administration, and the ability of letrozole administered after 5 years of tamoxifen to improve disease-free survival. Most recently, Cooperative Group studies have contributed to the development of a molecular profiling test, Oncotype Dx, which identifies women who have an excellent prognosis with hormonal therapy alone. Ongoing phase 3 clinical trials address the following questions: Is prolonged duration of aromatase inhibitor (AI) therapy beneficial? What is the efficacy and toxicity of steroidal versus nonsteroidal AIs in adjuvant treatment? Is combination hormonal therapy with an estrogen receptor down-regulator (fulvestrant) and an AI superior to an AI alone in the treatment of metastatic breast cancer? Does ovarian suppression offer superior benefit to standard therapy in the treatment of premenopausal breast cancer? What is the role of chemotherapy for early stage breast cancer selected via molecular profiling analysis? How can targeted therapies be used effectively in combination? Studies in subsets of patients defined by molecular profiling will be necessary to fully define breast cancer subtypes and realize the promise of personalized medicine. Close research partnerships that promote large-scale translational research are essential to the continuation of rapid achievements in this field.