Over the last 3 decades, a steady shift has occurred in the management of breast cancer. Because it was traditionally viewed as a local disease, many advocated the use of radical surgery to achieve maximum survival benefit. This view has been slowly replaced by a broader biologic view that recognizes the often systemic nature of breast cancer, even when it appears to be localized to the breast. Results from randomized clinical trials have demonstrated that less extensive surgery, or lumpectomy plus radiation therapy, are optimal for local management of early breast cancer. In addition to the less radical approach to surgical treatment of breast cancer, other randomized clinical trials have established the value of postoperative systemic therapy in improving overall survival by eradicating micrometastatic disease, the major cause of mortality from breast cancer. Despite the well-documented benefits of adjuvant systemic therapy, it is not effective in preventing death from breast cancer in all patients who are candidates for such treatment. The worth of such therapy can only be judged in retrospect upon disease relapse, a time when breast cancer is nearly always incurable. Currently, there are few reliable methods to predict the success or failure of a particular postoperative treatment modality, and better ways to predict and optimize outcome are needed.