Purpose To update the American Society of Clinical Oncology (ASCO)-Society of Surgical Oncology (SSO) guideline for sentinel lymph node (SLN) biopsy in melanoma. Methods An ASCO-SSO panel was formed, and a systematic review of the literature was conducted regarding SLN biopsy and completion lymph node dissection (CLND) after a positive sentinel node in patients with melanoma. Results Nine new observational studies, two systematic reviews, and an updated randomized controlled trial of SLN biopsy, as well as two randomized controlled trials of CLND after positive SLN biopsy, were included. Recommendations Routine SLN biopsy is not recommended for patients with thin melanomas that are T1a (nonulcerated lesions < 0.8 mm in Breslow thickness). SLN biopsy may be considered for thin melanomas that are T1b (0.8 to 1.0 mm Breslow thickness or < 0.8 mm Breslow thickness with ulceration) after a thorough discussion with the patient of the potential benefits and risk of harms associated with the procedure. SLN biopsy is recommended for patients with intermediate-thickness melanomas (T2 or T3; Breslow thickness of > 1.0 to 4.0 mm). SLN biopsy may be recommended for patients with thick melanomas (T4; > 4.0 mm in Breslow thickness), after a discussion of the potential benefits and risks of harm. In the case of a positive SLN biopsy, CLND or careful observation are options for patients with low-risk micrometastatic disease, with due consideration of clinicopathological factors. For higher-risk patients, careful observation may be considered only after a thorough discussion with patients about the potential risks and benefits of foregoing CLND. Important qualifying statements outlining relevant clinicopathological factors and details of the reference patient populations are included within the guideline. Additional information is available at www.asco.org/melanoma-guidelines and www.asco.org/guidelineswiki .