Sunlight produces many changes on our skin. Some of these we appreciate as cosmetically important, and some we see as medically destructive. Changes such as the appearance of wrinkling and skin cancer can come from the long-term direct effects of solar radiation. The sun has indirect effects on the skin which are mediated by disease processes, medications, immune reactants, and biochemical abnormalities. Understanding the nature of sun, how it produces its changes, and the wide variety of these manifestations is an important part of medical practice. By understanding the nature of sunlight, we are able to protect ourselves from its effects and to treat our patients.