The microcellular foams of polycarbonate (PC) were studied to gain an understanding of the effect that small bubbles have on the impact properties. The PC matrix was selected based on its intrinsic ductility. Sharply and bluntly notched samples were tested in Charpy impact configuration, so that the effects of the sharpness of the notch could also be studied. Sharply notched microcellular PC exhibited the most promising behaviour. The maximum load of foamed PC was substantially improved over that of unfoamed PC, Furthermore, the impact toughness (total energy absorbed) per unit thickness improved dramatically over that of the unfoamed PC. This effect is explained by the fact that the presence of small bubbles induces a transition from brittle to ductile behaviour by relieving the triaxial stress condition in front of the crack tip.