The association of cutaneous malignancy with degenerative changes of the dermis, in areas exposed to the sun, is frequently observed. This has led us to inquire into the nature of the relationship which exists between the two conditions. 1. Physiological Considerations As it is the intention of this paper to deal with conditions which arise from long-term exposure to solar radiation, it is necessary to consider certain physiological aspects bearing upon the main theme. The discussion will primarily involve responses to ultraviolet light, as other wavelengths seem to have no photobiological activity. Classification of Skin Types.—The first point is our own division of humans into three main types, depending on the degree of pigment in their skin, although all variations in grade between these types occur (Fig. 1). Type 1, the fair-complexioned person, has little or no natural (inherent) pigment