The impact strength of polycarbonate, exhibited over a wide temperature range, is due to the existence of overlapping dynamic mechanical loss peaks in this same temperature range. A dielectric analysis of the motions causing these peaks indicates they impart impact strength by providing a large number of degrees of motional freedom over which the energy of an impact may be dissipated. The magnitude of the impact strength is directly related to the magnitude of the dynamic mechanical dissipation factor measured under the same conditions.