It is estimated that 92,000 new cases of melanoma and 2,750,000 cases of nonmelanocytic skin cancer occur worldwide each year. Incidence of these cancers varies more than 100-fold from low rates in Asian populations to very high rates in the white population of Australia. Incidence of melanoma has been increasing in white populations by some 3% to 7% per year over the past 30 years; recent very sharp increases in some populations are probably due to early and increasing detection of cancers that were already there. Incidence of nonmelanocytic skin cancers probably is also increasing. Sun exposure is the main cause of skin cancer, accounting for at least 65% of melanomas worldwide and a much higher proportion in white populations. Pattern as well as amount of sun exposure is important in determining the risk of melanoma and probably also of basal cell carcinoma, with an intermittent pattern being associated with the greatest risk. There is increasing evidence that nonsolar sources of ultraviolet radiation, in particular sunlamps and sunbeds, increase the risk of melanoma, and PUVA therapy and exposure to ionizing radiation are established causes of nonmelanocytic skin cancer.