Since the early 1960s, ultrahigh-molecular weight polyethylene (UHMWPE) has been one of the most frequently and successfully used materials in total joint replacement. However, in the course of clinical usage, various weaknesses of the material became apparent including wear and aging properties. As a result of research on this polymer, several improvements in the quality of the raw material, the processing, and the sterilization have been introduced. These changes paired with an improved knowledge of the viscoelastic behavior of UHMWPE have led to an improved design of the components to meet the requirements of the given material characteristics and in turn to better clinical results. In the 1990s, investigations focused on the influence of radiation on the molecular structures of polyethylene. Gamma radiation treatment in air leads to an important decrease in the molecular weight with reduced tribological behavior and accelerated aging. In contrast, a nitrogen atmosphere during irradiation and storage can improve the linkage of the polyethylene and promote the positive effects of radiation treatment. This effect can be further intensified by increased radiation levels. Such highly cross-linked polyethylenes demonstrate extremely low wear rates in vitro;however, other material properties are also changed. First reports on fissures in highly cross-linked polyethylene explants demonstrate that only long-term clinical trials can allow a final verdict on the clinical potential of highly cross-linked polyethylenes.