The clinical and tribological history of metal-on-metal (MOM) hip bearings indicates mixed clinical performance. Low wear and excellent twenty-year clinical performance is reported, as is early loosening and high wear. Contemporary tribological studies of MOM hip implants combined with improved Co-Cr-Mo materials has led to more predicable wear performance, and provided an understanding of the dichotomy in historical clinical performance. Historical failures of MOM bearings can be attributed to one or more of the following: poor control of sphericity and radial clearances greater than 200 μm (high wear), poor implant design and/or cementing technique, inadequate radial clearance via matched head-cup pairs (seizing and high friction), and unpredictable Co-Cr-Mo microstructure and large carbides (two and three body wear). And conversely to these failure modes, the success of contemporary MOM bearings can be attributed to the following: modern machine tools are capable of reliably manufacturing MOM bearings with sphericity of +/- 3 to 5 μm, linear head penetration wear rates of about 5 to 20 μm per year (million cycles) are reliably achieved, and contemporary high carbon (0.25% C) wrought Co-Cr-Mo alloys possess homogenous microstructures with carbides less than 5 μm in size. Furthermore, contemporary radial clearances of 35 to 50 μm result in maximum Hertzian contact stresses of about 25 to 50 MPa, which is 50 times less than the compressive yield strength of Co-Cr-Mo (2700 MPa) or about 20 times less than the tensile yield strength. Therefore, mechanical damage due to excessive contact stress probably does not contribute to bearing failure. Rather, the optimum radial clearance is related to an optimized balance between maintenance of fluid film lubrication and avoidance of bearing seizure due to interference of opposing asperities and/or entrapment of third-body particles. The literature suggests 25 to 45 μm as the optimum radial clearance for 28 and 32 mm diameter head-cup components.