Breast cancer patients undergoing a mastectomy or breast-conserving procedure are candidates for axillary sentinel lymph node dissection (SLND) if there is no palpable or clinical evidence of axillary lymph node involvement. The finding of breast cancer metastases in axillary lymph nodes changes the staging of the disease, predicts the rate of recurrence and survival, and results in adjuvant treatment with chemotherapy, hormone therapy, or radiation therapy. The standard axillary lymph node dissection (ALND) of level I and II nodes has significant morbidity, of which lifelong lymphedema is the most feared by patients. Using a combination of radionuclide and dye injections, the correlation of SLND and standard ALND in finding positive lymph nodes is quite high (95%) in the hands of an experienced surgeon. Although at least one sentinel lymph node can be identified in the majority of cases, in a small percentage, identification may not be possible, necessitating complete axillary node dissection. In addition, a false-negative finding occurs in 3% to 10% of the patients having SLND—that is to say, the sentinel nodes are negative, but higher nodes are found to be positive. The advantages of SLND are the fewer complications versus ALND and the ability to identify sentinel lymph nodes that are not in the traditional level I or II areas. The identification of sentinel lymph nodes focuses the histopathologic examination, which may include immunohistochemical staining as well as the traditional hematoxylin and eosin (H&E). The importance of micrometastases (<2 mm) is under study; however, the total number of nodes involved with metastases may influence the adjuvant therapy that is offered. Contraindications to SLND include suspicious, palpable axillary lymphadenopathy; and regional breast operations (e.g., breast reduction) that alter normal lymphatic flow. Sentinel lymph node biopsy may be considered after prior axillary surgery, but lymphatic mapping may be necessary to identify alterations in drainage patterns and the identification rate of sentinel nodes may be lower.