Sentinel lymph node biopsy (SLNB) was introduced in 1992 to allow histopathologic evaluation of the "sentinel" node, that is, the first node along the lymphatic drainage pathway from the primary melanoma. This procedure has less risk of complications than a complete lymphadenectomy, and if the sentinel node is uninvolved by tumor the likelihood a complete lymphadenectomy would find metastatic disease in that nodal basin is very low. SLNB is now widely used worldwide in the staging of melanoma as well as breast and Merkel cell carcinomas. SLNB provides safe, reliable staging for patients with clinically node-negative melanomas 1 mm or greater in thickness, with an acceptably low rate of failure in the sentinel node-negative basin. Evidence-based guidelines jointly produced by ASCO and the Society of Surgical Oncology (SSO) recommend SLNB for patients with intermediate-thickness melanomas and also state that SLNB may be recommended for patients with thick melanomas. Major remaining areas of uncertainty include the indications for SLNB in patients with thin melanomas, pediatric patients, and patients with atypical melanocytic neoplasms; the optimal radiotracers and dyes for lymphatic mapping; and the necessity of complete lymphadenectomy in all sentinel node-positive patients.