Fluorescence spectroscopy has demonstrated a way to characterize and quantify skin aging and photoaging. The Fluorolog® allows direct in-vivo monitoring of endogenous skin fluorescence, which makes it possible to follow markers (tryptophan and collagen) in skin, and their changes in both healthy and diseased tissue. It also provides the possibility of evaluating the effect on skin from the modifications of these native proteins or the induction of The Fluorolog® will find a wide range of applications in skin research and photo-dynamic therapy.