Vitamins are essential nutrients, which must be supplied exogenously. They are organic compounds with indispensable biological activities as coenzymes in a multitude of cellular metabolic processes. Vitamin A, retinoids (vitamin A-derivatives), carotenoids, vitamin D, vitamin E, and vitamin K are fat-soluble, vitamin C and vitamins of the B-complex are water-soluble. This is of importance for gastrointestinal absorption in oral supplementation as well as the transdermal penetration for topical applications.