The energy mobility method is presented and applied both numerically and experimentally to thin plates of high modal density. The energy mobility is de"ned as the ratio of the frequency averaged quadratic velocity at one point of a structure to the frequency averaged active power injected at one point of the structure. It can be calculated using only classic input and transfer mobilities. The property of energy additivity of the contributions of several external loads is shown to be a good approximation in the case of a single structure. To quantify the approximations on typical structures one is interested in, numerical simulations are presented on plates. A good agreement is found between the predictions using the energy mobility and the exact calculations. An experiment conducted on a plate with a real excitation con"rms that the prediction of the energy mobility approach can be quite satisfactory. Finally, a remarkable property for the energy mobility (contrary to standard mobility) is noted: the insensitivity to the presence of a mass heterogeneity at driving points. This property allows one to simplify considerably the characterization of industrial structures that have in general a lot of heterogeneities. The energy mobility concept is further applied to the vibrational behaviour of an assembly. For each subsystem the connection is described by adding injected power and coupling power into the energy additivity. The equality of the frequency averaged quadratic velocity and the power #ow balance at each coupling point allows one to calculate the coupling power at connection points with energy mobilities of uncoupled subsystems. A system of two plates has been used to compare the energy mobility calculation and the exact one given by the classic mobility method. The comparison shows that a connectivity factor must be introduced to keep the same formalism generally used with classic mobilities. A generalized de"nition for the energy mobility is then established.