The mechanical behavior of gel-spun, ultra-drawn, UHMW-PE fibers was investigated as a function of temperature, stress, and time under static and dynamic loading conditions. From a phenomenological point of view, two separate contributions to the deformation behavior could be distinguished, i.e., a reversible (viscoelastic) contribution and an irreversible plastic flow component. It was investigated whether or not this distinction can be rationalized on a molecular basis. The fibers were studied using static (creep) and dynamic mechanical analysis (DMA), dilatometry, and wide-angle x-ray scattering (WAXS). The results of the combined experimental observations are discussed in an attempt to relate the deformation behavior of highly oriented PE fibers to events occurring on a molecular scale.