SUMMARY.—Developments in recent years permit us to state the following principles which relate to percutaneous absorption. (1) The entire stratum corneum junctions as the rate‐limiting barrier in the skin. (2) For penetration from weak solutions, flux is directly proportional to the concentration of the penetrant in the solution which is presented to the cutaneous surface. (3) Flux increases as the solubility of the penetrant in the stratum corneum increases. (4) Flux increases as the mobility of the penetrant in the stratum corneum (diffusion constant) increases. (5) Flux decreases as the thickness of the membrane increases. (6) When concentrations in the presenting solution reach high values, flux is not proportional to concentration. (7) Flux through dermal tissue is much more rapid than flux through the stratum corneum.For substances which penetrate relatively rapidly, the major pathway through the skin for steady‐state conditions is directly through the cells of the stratum corneum. For very slowly penetrating substances, shunts, e.g., the appendages, may be the major pathway. Strongly polar and strongly non‐polar molecules probably travel via different “molecular” pathways.Even though many of these principles have evolved fromin vitroexperiments, it is reasonable to expect them to apply equally well toin viv