Status of the axillary lymph nodes is one of the most important factors impacting overall prognosis and treatment for breast cancer. The sentinel lymph node (SLN) concept for breast cancer has been validated and SLN biopsy should be considered standard of care for axillary staging in patients with clinically node-negative axilla given the decreased morbidity when compared with axillary lymph node dissection. Ongoing controversy includes use of SLN in patients with ductal carcinoma in situ, prior axillary surgery, multicentric breast cancer, and large breast cancers. Determining the optimal timing of SLN in patients undergoing neoadjuvant chemotherapy and the prognostic and clinical significance of micrometastases remain areas of research.