This paper reviews the percutaneous penetration of corticosteroids in vivo in man utilizing urinary excretion of 14C-hydrocortisone as the experimental method. In general corticosteroids are only minimally absorbed from most areas of glabrous skin. Increasing the amount that penetrates – by use of occlusion – and increasing concentration provides the most practical method currently available for increasing efficacy in treatment-resistant dermatoses. Some practical research problems relevant to topical corticoid efficacy are discussed.