Skin has been paid attention as a site of application of prescription drugs, over-the-counter drugs (non-prescription drugs) and cosmetics. Skin permeation and skin concentration of the compounds should be considered after topical administration, as well as their blood concentration to evaluate efficacy and safety. Since the evaluation of the amount of drugs permeated through skin is important for topically applied drugs, studies on the skin permeation has been greatly advanced. In addition, many reports proved that skin permeabilities of drugs could be predicted from physicochemical parameters of drugs. On the other hand, few reports have been found on the prediction of skin concentration of drugs. Furthermore, many experimented problems are left to determine the skin concentration of drugs: severe consume of human or animal skins, difficult removal of applied drugs from the skin surface, low drug extraction ratio from skin and low sensitivity to determine skin concentration of drugs, and requirement of long time measurement. Thus, fast and accurate measurement of skin concentration of applied drugs are urgently required. This report describes the relationship between skin permeation and skin concentration, and the prediction of skin concentration of drugs using skin permeation parameters of drugs.