The International Skin Imaging Collaboration (ISIC) is a global partnership that has organized the world's largest public repository of dermoscopic images of skin lesions. This archive has been used for 3 consecutive years to host challenges on skin lesion analysis toward melanoma detection, covering 3 analysis tasks of lesion segmentation, lesion attribute detection, and disease classification. The most recent instance in 2018 was hosted at the Medical Image Computing and Computer Assisted Intervention (MICCAI) conference in Granada, Spain. The dataset included over 10,000 images. Approximately 900 users registered for data download, 115 submitted to the lesion segmentation task, 25 submitted to the lesion attribute detection task, and 159 submitted to the disease classification task, making this the largest study in the field to date. Important new analyses were introduced to better reflect the difficulties of translating research systems to clinical practice. This article summarizes the results of these analyses, and makes recommendations for future challenges in medical imaging.