The tandem scanning reflected light microscope invented by Petran and Hadravsky provides real time vision in the confocal mode. It performs nondestructive optical sectioning of biological material to a depth that depends on the transparency of the tissue. This real-time facility adds a fourth dimension (time) to the study of living specimens. In the present work, human skin was explored in vivo, at the cellular level to a depth of 150 μm. Two different anatomical sites were investigated: the back of the hand and the volar aspect of the forearm. The stratification of the horny layers, the nuclei of the living keratinocytes through the whole epidermis and the capillary loops within the superficial dermis constituted clear reference points. These permitted thickness measurements of the stratum corneum and epidermis to an accuracy of 1 μm. This method of exploration provides a new challenge for dermatologists and cosmetologist