Collagen can be preserved during tens of thousands of years in bones and teeth under favorable conditions. Natural isotopic abundances in carbon (13C/12C) and in nitrogen (15N/14N) of ancient bone and tooth collagen correspond to those recorded during the biogenic synthesis and have not been significantly altered during fossilization. These isotopic abundances are linked to those of the proteic fraction of animal and human diets, and to physiological conditions. Three kinds of applications are made possible through these natural isotopic signatures: determination of subsistence strategies in ancient human populations, determination of the diet of extinct species and the analysis of past environmental changes.