Skin diseases differ from those of internal organs in that the changes they produce are visible to both patient and doctor. Every minor blemish can be seen, whereas similar changes in the lungs or stomach would probably pass unnoticed. This accounts for the great diversity of skin diseases; those that are named run into thousands and many of these are capable of subtle variation in appearance. To the novice this array is confusing, especially as many are named in elegant Latin phrases. However, research over the last century has clearly defined many important groups on the basis of their cause. There are, for example, those due to bacterial, virus, or fungus infections, those due to irritating substances or to allergy of the skin, cancers of the skin, and those associated with internal diseases. Most of these conditions are capable of rational treatment. Despite all the advances in our knowledge the causes of many common skin diseases are unknown and will remain unknown until some of the fundamental problems of medical science are solved.