Relaxation rate values depend on the time-dependent interactions present between the observed nuclei and the surrounding. In fact, the fluctuations in the energy of these interactions can determine transitions between spin states and thus allow for the recovery of the equilibrium conditions. Therefore, the evaluation of the nuclear relaxation rates can permit to obtain information on the interactions occurring in the system and on their dynamics (like molecular reorientation, translational diffusion, chemical exchange…). Each type of interaction can be the source of a different relaxation mechanism and can be modulated by a different motional process. In many cases, the measured relaxation rates are the sums of the contributions arising from many mechanisms. Each relaxation mechanism has a specific dependence on the applied magnetic field and on the relevant time constants of fluctuations. In many cases the measurement of the relaxation rates at a single magnetic field does not provide enough information to recover the relaxation mechanisms and all parameters used to describe them.