Communication between cells as a prerequisite for life of multicellular organisms is achieved mostly by solvent mediators like hormons, neurotransmitters, cytokines and chemokines. Increasing evidence suggests that a class of cytokines exists that signals from immune cells specifically towards epithelial cells. The most prominent member of these "tissue-signaling cytokines" is Interleukin-17 (IL-17). This book illustrates IL-17 is essential to protect the human organism at its skin surface against invading microorganisms. Cellular sources of IL-17 and trigger factors of IL-17 secretion in the skin are defined. Finally, this book identifies two human diseases associated with chronic infections of the skin that are at least partially caused by a disturbed IL-17 pathway – the orphan syndrome "chronic mucocutaneous candidiasis" and the very common inflammatory skin disease "atopic dermatitis/ eczema".