Two key words characterize the uniqueness of skin lipids: complexity and perversity. Each suggests a function. Complexity manifests itself in the large number and variety of both saturated and unsaturated fatty chains synthesized by human skin. Functionally, this allows each individual to have a distinct odor or chemical fingerprint. Perversity manifests itself when one compares the lipids synthesized by skin with those synthesized by internal tissues. For example, skin makes odd instead of only even chains, branched instead of only straight chains, free instead of only esterified acids, places double bonds in unusual positions in the fatty chains, extends chains to extreme lengths, and accumulates intermediates in the synthesis of a biologically valuable compound such as cholesterol. Functionally, these products may pose metabolic problems to potential pathogens and thus contribute to the survival of only compatible microorganisms.