Treatment selection for breast cancer is still largely empiric and guided by large randomized clinical trials on populations of patients. This approach is inadequate for the selection of individualized chemotherapy regimens. Estimates of benefits for individuals are extrapolations from the effects seen in these large trials, and do not necessarily apply to individual patients. The revolution in genomics promises to transform oncology care. By better defining cancer subtypes, a better understanding of breast cancer biology should help to guide treatment.