Polymeric materials do not always possess mechanical properties that can comply with engineering purposes. Most thermoplastics have a low impact resistance that may, however, be improved (1–4), particularly by the addition of rubber particles (5–7). This strategy is by far more effective than the addition of a plasticizer that is detrimental to the glass transition temperature (T g), modulus, and yield strength. The rubber particles are responsible for stress concentration that results in multiple crazing (8,9) or multiple shear deformation, depending on the matrix (10–13). The rubbery phase contributes only marginally to the toughness of the blend (9). The major disadvantage of the rubber modification of thermoplastics is the loss in stiffness (modulus and yield stress). This is the reason why much attention has been paid to optimizing parameters such as the rubber phase volume fraction, rubber particle size, distance between particles, and adhesion between the rubbery particles and the matrix. Wu (for semicrystalline polymers) (10,11) and Meijer and coworkers (for amorphous polymers) (15–17) proposed that a transition occurs from crazing to shear yielding when the matrix ligaments between the rubber particles or the characteristic thickness in layered structures are below a critical size. Therefore, the local deformation mechanism depends on whether the polymeric material is made locally thin, which means that the interparticle distance (ID) in rubber modified thermoplastics is of crucial importance. Meijer and coworkers (15–17) were able to correlate the aforementioned experimental structure-property relationship to the intrinsic behavior of the polymer matrices to be toughened. It was proposed that toughness could be improved by only acting on the intrinsic postyield behavior of the polymer, i.e., strain softening and hardening. The intrinsic deformation behavior has to be determined by methods not sensitive to localized phenomena (i.e., necking and crazing), e.g., by uniaxial compression testing (14). The intrinsic stress-strain curve shows an initial elastic region followed by yielding, strain softening, and strain hardening. The intrinsic difference between polystyrene (PS), known as a brittle material, and polycarbonate (PC), known as a ductile material, can be found in the postyield behavior: first, a drop in true stress after yielding (strain softening that induces strain localization) and, second, the increased slope of strain hardening at large strains (that stabilizes the localized plastic zone). A moderate strain softening and strong strain hardening should yield ultimate toughness because the moderate localization due to strain softening could thus be easily stabilized by strong strain hardening.