A research program directed toward gaining an understanding of the molecular mechanisms of large strain deformation in glassy and crystalline polymers has been carried out. The investigation encompassed two types of experimental procedures: deformation experiments on bulk polymers to determine the thermal activation parameters of the rate limiting step of the deformation process, and microscopic examination of deformed samples to determine what types of molecular motions had taken place. Macroscopic deformation experiments were performed of polycarbonate and polyethylene. Activation enthalpies, shear activation volumes, and dilatation activation volumes were determined for each polymer. Microscopic studies were performed on extended chain crystalline polyethylene which had a spherulitic structure. Deformation was found to occur on discrete shear bands. The spherulitic structure did not seem to strongly affect the deformation. Microscopic examination revealed that deformation could take place by intralamellar slip in any direction on any plane parallel to the direction of the molecules, by interlamellar shear, and by formation of kinks. The behavior of shear bands in polycarbonate is also discussed, and a technique for producing metallic replicas of polymer surfaces is described.