Malignant melanomas of the skin are becoming more common. Earlier diagnosis has led to better individual prognoses, but this has not prevented the death rate in the population from rising. This paper brings up-to-date studies on the etiology of malignant melanoma, and supplements various fuller but earlier reviews. Melanoma risk is increased in lightly pigmented people who burn in the sun and do not tan well. People with an increased number of nevi are also at increased risk. Latitude of residence is important, risk in white populations increasing with distance from the poles. Phenotypic factors can over-ride location, so that Mediterranean people have lower rates than Scandinavians. Melanomas are concentrated on sites exposed by clothing, but this concentration is not as strong as for the other skin cancers. Migrants to sunny climates are at less risk than similar people born locally, provided that they migrated as adults. Outdoor work carries only a small excess risk of malignant melanoma, in contrast to the other skin tumors. Melanomas are commoner, and kill more often, among people of high socioeconomic status than low. The two most detailed contemporary reconciliations of these pieces of evidence are based on the ideas that intense brief exposure is the critical factor, or that exposure in childhood is of major importance. With the expected changes in the global concentration of ozone in the stratosphere, it is necessary to estimate the relationship between the intensity of sunlight and melanoma risk.