By 1927 for basal cell carcinoma (BCC) and squamous cell carcinoma (SCC), and by 1955 for melanoma, the broad grounds for relating sun exposure to skin cancer had been established: that these are more frequent in residents of areas of high ambient solar irradiance, are more frequent in sun-sensitive people, occur mainly on sun-exposed body sites, are more frequent in people with high sun exposure, and are more frequent in people with benign sun-related skin conditions. The past 40 years have added both quantity and quality to the epidemiological evidence and, most recently, provided direct evidence that sun exposure is the cause of mutations in critical tumour suppressor genes in BCC, SCC and melanoma. Complete or more convincing answers are still needed to many questions of detail. They include whether the pattern of sun exposure is really important in, and acts independently of amount of sun exposure in, affecting the risk of melanoma and BCC; what the shape of the relationship between the amount of sun exposure and risk of BCC and melanoma is when the pattern of exposure is held constant; whether there really is a plateau in risk of BCC and melanoma beyond some level of the amount of exposure; whether this exposure-response relationship depends on cutaneous sensitivity to the sun and in what way; whether sunburn makes a specific contribution to the risk of skin cancer independent of the amount of sun exposure; whether sun exposure close to the time of diagnosis of skin cancer contributes anything to the development of the cancer; what the solar radiation action spectrum is for each kind of skin cancer; and whether sunscreens are effective in protecting against skin cancer.