We describe some aspects of NMR in a porous medium that are of interest to both petrophysicists and log analysts, that is, measurements that are useful for characterizing porous structures and for determining the distribution of fluids in a porous volume. The principles for interpreting NMR saturation logs follow from these considerations. Our analysis is based essentially on a thorough study of the literature, with particular emphasis on petroleum, and on recent work carried out at the Institut francais du petrole.