Melon (Cucumis melo L.) presents extensive diversity, particularly at the fruit level. The combination of independent phenotypic traits such as sex expres- sion, fruit shape and size, skin colour, flesh colour, presence of gelatinous sheath around the seeds, and seed size can be used to define horticultural Groups and Sub- groups. This chapter presents a brief description of 19 Groups of wild, feral and domesticated melons, and for some of them of Sub-groups. Two new Groups are proposed: “kachri” for accessions intermediate between wild and cultivated, and “indicus” for certain accessions cultivated mainly in central India. It is proposed to merge the “cantalupensis” and the “reticulatus” Groups, and to split the large “inodorus” Group into three Groups, namely “cassaba”, “ibericus” and a smaller “inodorus”. A key for the determination of the Groups is proposed.