AbstractSemicrystalline, as well as amorphous, polymers frequently develop some measure of molecular mobility at temperatures well below their melting temperature, Tm, or their glass transition temperature, Tg. In these polymers, one or more damping or internal friction peaks are usually found to occur in the low temperature region from 4°K to 300°K. The magnitude, and temperature position, of these maxima are dependent upon the structure of the polymer, its mode of preparation and past history, and the local environment of the moving units. For purposes of discussion, the observed relaxation processes may be classified, on the basis of their temperature location at low measuring frequencies (∼1 Hz) as δ‐processes, those occurring below 80°K; γ‐processes, those occurring in the temperature range from 80°K to about 240°K; and β‐processes, those occurring from about 200°K to above 300°K. Various examples will be given of polymers that show significant δ, γ, and β‐processes and the influence of structure, structural substitution, crystallinity and low molecular weight diluents will be illustrated.The extent to which relaxation behavior can be correlated to macroscopic ductility and fracture toughness is discussed. It is noted that the lowest temperature δ‐relaxations arise primarily from reorientational and/or wagging motions of pendant methyl, ethyl, and phenyl side chains and, as such, have little bearing on macroscopic impact strength. The intermediate temperature γ‐processes arise from reorientational motions, of short side chain sequences or from local motion of main chain sequences. Some degree of main chain mobility. appears to be an essential requisite if ductility and toughness are to be realized. The higher temperature β‐processes in the 200°K t o 300°K range arise from motion of polar side groups, from motion of short chain sequences involving water complexes, or from large‐scale motions associated with the Tgof the amorphous regions. In the latter instance, the β‐process i s accompanied by a large increase in ductility and impact strength and this feature is utilized in polyblends and copolymers to raise the fracture t