Predictive factors are those factors that predict the effects of chemotherapeutic or other agents on the tumor or the host. Many factors have been investigated for their predictive value of the effect of chemotherapeutic or hormonal agents. Estrogen or progesterone receptors are the most established predictive factors for hormonal therapy. Her-2/neu is a predictive of the effect of the monoclonal antibody trastuzumab and of certain chemotherapeutic agents. Other predictive factors remain under clinical investigation. Most cases of breast cancer are initially considered to be systemic disease. Cure can only achieved with surgery that leaves no residual cancer cells, followed by an appropriate form of systemic therapy. In the clinical situation, local therapy and systemic therapy for breast cancer have been considered independently. However, preoperative chemotherapy has become common recently. The interaction between chemotherapy and surgery should be considered because the results of preoperative chemotherapy affect the choice of operative technique. Predictive factors for the effect of radiation therapy should also be taken into account after breast-conserving surgery. It remains to be determined which predictive factors should be considered at which time.