Transdermal penetration of drugs is currently much studied because it presents several attractive aspects, avoiding first past effects and offering means of immediate breaking of the therapeutic in case of hitch. But it must overcome the very efficient hindrance of the epidermal barrier. Few among the molecules have prerequisite structural and pharmacodynamical aspects necessary to this way of penetration. Substances belonging to diverse chemical families enhance sometimes largely this passage; they are named "penetration enhancers". In this review are listed the ideal properties of enhancers, the chemical structure of the currently more used ones, the possible interaction with other components of the formula in the case of TTS and towards the patient.