The most powerful predictor of survival in breast cancer is the presence or absence of lymph node metastases. Lymphatic mapping and sentinel node biopsy is a new technique that provides more accurate nodal staging compared to routine histology for women with breast cancer without the morbidity of a complete lymph node dissection. Sentinel lymph node biopsy is a more conservative approach to the axilla that requires close collaboration between the surgical team, nuclear medicine, and pathology. National trials are investigating the clinical relevance of the upstaging that occurs with a more intense examination of the sentinel node. Since complaints due to the axillary node dissection are a common occurrence after definitive breast cancer surgery, if the side effects of the level I and II node dissection can be avoided, particularly in the node-negative population, a major advance in treating this disease will be made.