This review deals with difficulties of diagnosis in cutaneous malignant melanoma encountered by histopathologists of variable seniority and is based on referred material at The Royal Marsden Hospital over a 20-year period and on the experience of more than two-and-a-half thousand cases referred to The World Health Organisation Melanoma Unit which I reviewed when chairman of the Pathologists' Committee. Though there is reference to the differential diagnosis of primary and metastatic tumour, the main concern is with establishing the diagnosis of primary melanoma to the exclusion of all other lesions. An appendix on recommended diagnostic methods in cutaneous melanomas is included.