Skin protects the human body from dryness and its molecular mechanism has been studied by both biological and biochemical approaches. The epidermis is the outer layer in skin and forms a water barrier. Animal experiments with diets deficient in essential fatty acids have demonstrated that lipids containing linoleic acid are essential molecules in the maintenance of the epidermal water barrier. Structural analysis revealed that linoleic acid-containing epidermal lipids include acylglucosylceramides, which are comprised of both very long chain ω-hydroxy fatty acids as the amide-linked fatty acids and linoleic acids as the esterified fatty acid linked through the ω-hydroxyl group. Human epidermis consists of two acylglucosylceramide groups, one containing sphingenine and the other containing phytosphingosine with a single double bond. We named these functional lipids“Epidermosides”. ω-Hydroxylation is the rate limiting biosynthetic step for the expression of these functional lipids and investigation of the hydroxylase active in this process is an important target for the further research.