Sentinel lymph node (SLN) biopsy has replaced routine axillary lymph node dissection (ALND) for most breast cancer patients with clinically normal lymph nodes. The morbidity (lymphedema, arm numbness) of SLN biopsy is significantly less than ALND. The use of alternative injection sites (skin or subareolar) yields high SLN identification rates and may shorten the learning curve associated with standard peritumoral injection. The dual-agent (radiocolloid plus blue dye) technique is recommended to decrease false-negative rates, especially when surgeons are just learning how to perform SLN biopsy. Regardless of the technique employed, SLN identification rates should be > 95% with a false-negative rate of < 5%. Using serial sectioning and immunohistochemistry, SLN micrometastases can be identified in 10% to 20% of node-negative patients. However, the clinical significance of micrometastases is not known. Axillary recurrence is rare for patients without SLN metastases who do not undergo further axillary surgery. Outside a clinical trial, ALND is recommended for most patients with SLN metastases, except for cases with SLN metastases < 0.2 mm detected by immunohistochemistry alone. The indications for SLN biopsy have expanded and include breast cancer patients with multifocal/multicentric disease and large tumors, and male breast cancer. Although minimally invasive internal mammary SLN biopsy is feasible, the usefulness of this procedure is not established.