The mechanism of the relaxation of water in the presence of paramagnetic or superparamagnetic species, which are used as contrast agents for magnetic resonance imaging, is described. A particular focus is set on the analysis of the nuclear magnetic resonance dispersion profiles obtained on aqueous solutions of these species, with a description of the most important parameters that can be extracted from this analysis. These are very important since they allow one to tune the structure of the contrast agents in order to try to improve their efficacy.