A better understanding of the influence of the metallurgy and tribology on wear of the component and improved manufacturing technology have allowed the re-introduction of metal-on-metal articulations in total hip replacement. Excellent early clinical outcome has been reported with the new generation of metal-on-metal components. Retrieval studies of previous metal-on-metal bearings and contemporary implants have shown low rate of wear. These hard bearing surfaces are especially promising for young and active patients. Although metal-on-metal bearings produce significantly less wear debris than metal-on-polyethylene bearing surfaces, the debris generated results in the body being exposed to metal ions for prolonged periods. Concerns exist over metal hypersensitivity, osteolysis, chromosomal mutation, carcinogenicity and fetal exposure to high ion levels. These matters require further investigation, particularly the long-term exposure in younger patients and the recognition that some patients may have unusually high ion levels. From tribological studies, joint simulation testing and clinical trials, it has been proposed that the extent of wear can be reduced by using components with a larger diameter with a high carbon content in the alloy (0.2%), well-adjusted clearance for the components, better implant sphericity and lower surface roughness.