The Multicentre Selective Lymphadenectomy Trial – 3 (MSLT-3) is a phase III, International Multicentre, Randomised Controlled Trial of Selective Index Lymph Node (ILN) Resection versus Therapeutic Lymph Node Dissection (TLND) After Neoadjuvant Immunotherapy for Stage IIIB-D Melanoma. A total of circa 1500 patients will be randomised to ILN resection or TLND. All patients will have a magnetic, radioactive, or sonographically visible marker placed in the largest metastatic lymph node (= Index Lymph Node [ILN]) at baseline, prior to the start of NAT. All patients will receive 6 weeks of standard NAT (anti-PD1 immunotherapy based), then undergo the randomly assigned surgical procedure. In the ILN arm, only the index node will be removed through a targeted resection (similar to a post-chemotherapy sentinel node procedure in breast cancer). The pathological response to immunotherapy will be assessed according to the International National Consortium classification (INMC)[13]. Patients with a major pathologic response (MPR) (defined as ≤10% of viable tumour cells in the node nodes) in each surgical arm will form the analysis group for the study objectives. Patients with a MPR in the ILN arm will have no further surgery and those without a MPR in the ILN arm will have a TLND in a second procedure. All patients will be followed-up with periodic imaging and physical examinations for disease surveillance for up to 10 years.