The most important function of the skin is to form a barrier between the body and the external environment. The epidermal barrier prevents transepidermal water loss from the skin, but also serves as a barrier to the entry of harmful environmental allergic, toxic or infectious substances. Inherited defects in the genes encoding the components of the epidermal barrier result in the development of rare genetic disorders, whereas polymorphisms in these genes together with environmental factors cause frequent inflammatory skin diseases, such as atopic dermatitis. In this review, components of the skin-barrier function will be reviewed with special emphasis on how the altered epidermal barrier might be repaired. The different strategies to increase the transdermal penetration of drugs is also discussed.