The skin is the boundary between ourselves and the world around us. It is an important sense organ, and controls heat and water loss. It reflects internal changes and reacts to external ones. Usually, it adapts easily and returns to a normal state, but some-times it fails to do so and a skin disorder appears. Some of the internal and external factors that are important causes of skin disease are shown in Fig. 1.1. Often several will be operating at the same time; just as often, no obvious cause for a skin abnormality can be foundaand here lies much of the difficulty of dermatology. Nevertheless, when a cause is obvious, such as the washing of dishes and the appearance of irritant hand dermatitis, or sunburn and the develop-ment of melanoma, education and prevention are just as important as treatment.