1. Screen and recruitment: patients will be screened based on their prior history and intention to initiate treatment with an immune checkpoint inhibitor. 2. Study phase: * Collection of baseline demographic data, prior disease history, nature of ICI therapy, outcome data of ICI therapy (treatment disposition, toxicity, tumor response and survival). * Collection of blood samples will be performed every 3 to 4 weeks for the first year on ICI therapy and every 6 to 12 weeks thereafter until the end of ICI therapy, disease progression, death or loss to follow-up. Collection of blood samples will be aligned with the visits necessary for the administration of the ICI therapy. * Collection of archival melanoma metastasis tissues will be performed on a continuous basis and be triggered by availability of such tissue following therapeutic resections of melanoma metastases. 3. Follow-up phase