The study of the biological effects of sun on the skin is one of the most topical questions in the recent dermatological literature. Interest in these effects has grown since it was demonstrated that the sun accelerates intrinsic skin ageing and is a principal factor for skin cancer. Skin damage caused by the sun is mainly due to UV radiation. Skin damage certainly has ancient roots, but has undergone sudden changes since man began to migrate to different geographical areas, for example when northern European populations colonised sunny areas close to equator. It is not a coincidence that the highest incidence of sun induced neoplasias is observed among white population of Australia. This epidemiological finding focused the interest towards the identification of phenotypic factors conditioning skin response to sunlight, and hence towards the definition of the so called phototype. After the fundamental work of Fitzpatrick based on sun exposure history more recent studies have shown that skin response to UV-rays can be predicted, to a good approximation, by skin colorimetry. Therefore this simple, cheap and non invasive measurement enables to predict sun reactivity skin type and to evaluate the melanoma risk.