Vitamin D synthesis is initiated by conversion of cutaneous 7-dehydrocholesterol (7-DHC) to previtamin D after exposure to solar ultraviolet radiation (UVR). Thereafter, previtamin D undergoes non-photobiological steps to become biologically active vitamin D. The whole process, which is the only well-established benefit of solar UVR exposure, depends on many factors including genetics, age, health, and behaviour. However, the most important factor is the quantity and spectral quality of UVR reaching 7-DHC in skin. Vitamin D synthesis specifically requires ultraviolet B (UVB) radiation (~295-315nm in sunlight) that is the minority component (<5%) of solar UVR. This waveband is also the most important for the adverse effects of solar exposure, including sunburn (erythema) and epidermal DNA damage that is a prerequisite for most skin cancers. The properties of UVB at the Earth’s surface depend on many physical and temporal factors such as latitude, altitude, season, time of day and weather. The quantity and spectral quality of UVB reaching 7-DHC also depend on personal, cultural, and behavioural factors. These include epidermal thickness, skin melanin, clothing, body surface area (BSA) exposed, holiday habits, and sunscreen use. There is considerable disagreement in the literature about the role of most of these factors. New recently published data on these controversies will be presented including wavelength dependence (action spectroscopy), BSA, melanin and sunscreen use. Accurate action spectroscopy is essential for risk-benefit analyses from solar exposure. It can be argued that vitamin D supplementation obviates the need for solar exposure, but many studies have shown little benefit from this approach for a wide range of health outcomes. In any case, public health advice must optimize risk versus benefit for solar exposure. It is therefore fortunate that the individual UVB doses necessary for maintaining optimal vitamin D status are much lower than those for sunburn, irrespective of skin melanin.