The paper presents the first systemic classification and analysis of transdermal penetration enhancer structures from the point of view of medicinal chemistry. The mechanism of effect of these compounds is not based on a receptor interaction, nevertheless we have found some structural regularities of the enhancing effect. Molecules of the most effective enhancers nearly always contain a fragment of the natural moisturising factors present in human skin at normal physiological conditions. The simplest structural fragment of efficient enhancers is X-CO-N= where X is -CH₂-, -NH₂, -NH-, and it shoved contain a long alkyl or alkenyl (C₈-C₂₀) chain as well, including possible isosteric modifications. The d-amins acid derivatives as the substances complying fully with these criteria, have been intensely studied.