Sunshine is one of mankind's oldest remedies. Merely forgotten, UV light treatment came back recently due to (1) technological progress, providing particular UV wavelengths (2) basic research, discovering mode of action at molecular level, and (3) rising knowledge about skin as the primary target. Human skin is a rather difficult organ, possessing a considerable part of the immune system, the so-called skin associated lymphoid tissue. Different biological effects are caused by UV irradiation at different wavelengths. UV radiation alone as well as together with a photosensitizer also can cause systemic effects. Dealing with these, results of basic research and their application on modern treatment of some skin diseases, transplantation medicine, non-specific resistance system, and treatment of cardiovascular diseases are discussed.