The history of impact testing of plastics is reviewed and the current theories of impact resistance are discussed. The effect of velocity on the energy absorbed by polymethyl methacrylate at various temperatures has been studied with the aid of two new testing machines. The results are interpreted in terms of the classical theories of mechanics of materials and the current theories of the rheology of high polymers. It is concluded that there are two distinct critical velocities of straining in this material—the lower one corresponding to the relaxation of the bonds restraining chain-chain slipping and the higher one corresponding to the relaxation of the secondary bonds involved in chain uncoiling. Both of these relaxations are temperature dependent—the lower one to a slight extent and the upper one to a marked extent. Above the two critical velocities the response of the material is purely elastic. A method of calculating the energy to break at rates of straining above the second critical velocity from the slow-rate stress-strain curves is demonstrated.