The characteristics of chronologically aged skin should be differentiated from the features of photoaging, which is marked by yellowed, leathery, sagging, wrinkled, elastic skin, as well as underlying connective tissue damage and various benign, premalignant, and malignant neoplasms. Laboratory experiments with the hairless mouse exposed to varying doses of ultraviolet radiation have demonstrated the protective effects of sunscreens, the repair of ultraviolet damage when the skin is no longer bombarded by photons, and the enhancement of that repair by retinoids.