Metal-on-metal hip resurfacing arthroplasty (MoMHRA) is an alternative treatment option to total hip arthroplasty (THA), especially in the young and active. Despite the early successful results, reports of adverse soft-tissue reactions around the MoMHRA have caused increased concern. These reactions have been termed pseudotumours and have been shown to be associated with increased wear. However, their incidence and aetiology is not known. The broad aims of this thesis were to 1) determine the clinical impact of pseudotumour; and 2) identify ways of reducing the risk of pseudotumour. The clinical impact was determined by examining the incidence and prevalence of pseudotumours needing revision and the subsequent outcome following revision. At the Nuffield Orthopaedic Centre, the 10-year survival of the BHR, commonest MoMHRA used, was 87.1% (95%CI: 83.0% to 91.2%) and the 10-year incidence of revisions due to pseudotumour was 7.5% (95%CI: 4.3% to 10.7%). In a prevalence study of two tertiary referral centres, pseudotumour was identified as the most common revision indication. An outcome based, case-control study characterised outcome following revision due to pseudotumour as inferior to that following revision for any other failure mode, and inferior to the outcome of matched primary THAs. The inferior outcome was associated with the extensive tissue damage at revision. Risk factors contributing to development of pseudotumour were found to be both patient and surgeon related. Patient factors significantly increasing risk included female gender and small component size. Case control studies demonstrated two surgical factors that were associated with significantly increased risk; cup malorientation and a reduction in head neck ratio (HNR) at operation particularly in females. Furthermore, evidence based recommendations were provided on optimum cup orientation and screening practice. Surgeons should aim for a radiographic cup inclination/anteversion zone of 45°/20° ± 10°. Patients with mal-positioned cups, those with high HNR prior to surgery, and those that had significant HNR reduction with the MoMHRA should be considered at increased risk of developing a pseudotumour. An ex vivo study demonstrated that the majority of pseudotumours occur due to high prosthesis wear, and result from an exaggerated innate response with an accompanying variable immune response. However a small but significant number of pseudotumours (20%) were observed with low wear and a prominent immune response. Furthermore, the cups oriented within the optimum zone were associated with less wear. Hence, subsequent in vivo research focused on providing evidence-based guidance on how to achieve the target cup orientation and factors to consider in minimising the scatter in cup orientation observed following hip arthroplasty. This great scatter was mostly due to two factors: 1) the variability between surgeons at orientating a component at implantation and 2) the difference in the pelvic position between implantation and radiographic assessment. The angular offset was 5° for inclination and 8° for anteversion because of the different definitions at impaction and X-Ray. Surgeons should need to be aware of these factors. Greater amount of pelvic movement occurs during MoMHRA, in comparison to THA. To achieve the target MoMHRA cup orientation surgeons should implant the cup with an intra-operative inclination/anteversion of 40°/30°. It is concluded that pseudotumour following MoMHRA has a significant clinical impact. In order to minimise it, surgeons should appropriately select patients and optimise surgical technique to achieve optimum cup orientation and try to avoid reducing the native head neck ratio. Appreciation of risk factors for pseudotumour development (female gender, small size component, cup orientation, HNR reduction), would aid detection prior to extensive soft-tissue damage and improve inferior outcome associated with revision.