Patients who are young or active or both who require total joint replacement pose a unique challenge; their high activity demands wear-resistant bearings that will perform for decades, without suffering from the adverse effects of accumulated wear products. We discuss the tribologic and biologic properties of newly introduced bearing materials for hip prostheses. The new PEs are intended to address the aseptic loosening problem by reducing the volume of submicron PE particles to a level well below that historically associated with osteolysis. However, choosing among the several variations of the cross-linked thermally-stabilized PEs is confounded by conflicting opinions regarding the optimum balance between long-term wear resistance and mechanical strength, and regarding potential effects of differences in morphologic features of the submicron-sized wear particles on their relative osteolytic potential. Metal-on-metal bearings have clinically proven wear resistance and the advantage of self-polishing, but the long-term biologic effects of metallic ions remain unknown. Ceramic-on-ceramic bearings have the advantage of high biocompatibility and usually very low wear, but fracture remains a rare but catastrophic complication. The choice of an appropriate bearing couple should be made after a thorough consideration of the relative risks and potential benefits of each of these materials.