An important industrial property of a polymer is its impact strength, the ability to dissipate the energy of an impact, through some mechanical loss process, without breaking. This property may be enhanced through the judicious use of fillers and grafts, yet the reason why oneunmodifiedpolymer has a higher impact strength than another is not understood. It is anticipated that an understanding of this phenomenon will provide a firm foundation for the modification of polymers in order to improve their impact properties.