The major source of ultraviolet radiation is solar radiation or sunlight. However, exposure to artificial sources particularly through tanning salons is becoming more important in terms of human health effects, as use of these facilities by young people, has increased. The International Agency for Research on Cancer has noted that there is sufficient evidence from studies in animals and in man to establish ultraviolet radiation as a human carcinogen. Skin cancer has been the most commonly studied cancer site with respect to UV radiation. The nature and timing of sun exposure appear to be important determinants of both the degree of risk and the type of skin cancer. Cutaneous malignant melanoma and basal cell cancer are much more strongly related to measures of intermittent ultraviolet exposure (particularly those of childhood or adolescence) than to measures of cumulative exposure. In contrast, squamous cell cancer is more strongly related to constant or cumulative sun exposure. Lip cancer is causally related to lifetime sun exposure. It has been estimated that solar ultraviolet radiation accounts for approximately 93 percent of skin cancers and about half of lip cancers. This translates to approximately 4,500 life-threatening cancers (cutaneous malignant melanoma) per year in Canada, as well as 65,000 less serious cancers (basal cell cancer, squamous cell cancer and lip cancer). Appropriate clothing use, care not to sunburn and judicious use of sunscreens could prevent at least half of these and save approximately 450 lives per year. In addition, physician and public education programs can significantly increase the proportion of melanomas diagnosed early. Lesions that have not yet penetrated deeply are associated with a mortality rate of less than five percent. Several recent studies suggest a possible inverse relationship between ultraviolet radiation exposure and risk of non-Hodgkin lymphoma, colon, breast and prostate cancer, and investigators have speculated that this might be due to the higher serum levels of vitamin D stimulated by high lifetime sun exposure. Further, studies conducted within cohorts using stored pre-diagnostic serum suggest that those with high levels of vitamin D have lower incidence rates of a number of malignancies, particularly colon cancer. However, since serum vitamin D levels can be raised through the use of supplements without increasing risk for skin lip and other known UV-related cancers, changes to health policy with regard to exposure are not merited at this point. Further research is needed in this area.