Within the past 10 years, covalently bound lipids have been identified both in wool and hair and in epidermal stratum corneum. In hair, fatty acids, the most abundant of which is 18-methyleicosanoic acid, are attached via thioester linkages to protein at the outer surface of cuticle cells. This lipid layer, the fatty layer or F-layer, is thought to be of major significance in determining the surface properties of hair and wool. In epidermal stratum corneum, the covalently attached lipid has been identified as an omega-hydroxyceramide consisting of 30-through 34-carbon omega-hydroxyacids amide-linked to sphingosine bases. The hydroxyceramide molecules are attached to the outer surface of the cornified envelope through ester linkages, and provide a monolayer covering the outer surface of all corneocytes. This covalently bound lipid layer, or lipid envelope, interacts with free lipids and has a major influence on the organization of lipid lamellar structures in the intercellular space.