Total hip replacements have been in use for over 30 years, and have shown great improvement from design to surgery since the first generation of implants were introduced. The greater need for hip replacements has led to the development of test methods that can be applied in the lab, which can predict the lifetime of a particular implant. To achieve this aim this study has sought to investigate key parameters, which may affect wear and its subsequent effect upon the production of particles for various material combinations and bearing geometries, under high (jogging) and low (walking) loads, with concurrent assessment of wear reduction, particle size and morphology. The clinical use of cross-linked polyethylene (XLPE) has been shown to reduce wear and thereby the onset of osteolysis in total hip arthroplasty. Hip simulator studies have suggested that while XLPE generates low wear under smooth counterface bearing surfaces, there appears to be an increased sensitivity to scratched femoral head conditions which can occur in the patient. However, these simulator studies have not combined damaged articular surfaces with a severe gait model, representing the worst-case scenario for high-risk, active patients. This hip simulator study has shown that the size distribution of wear particles generated in tests on 5 MRads crosslinked polyethylene can be influenced by the degree of patient activity. Fast jogging showed a greater influence on the number of sub-micron-sized wear particles (5-fold increase compared to walking) than on volumetric wear rate (26 mm3/106 cycles compared to 29 mm3/106 cycles). Fast jogging also did not generate the largest wear particles (>I 0p m) produced by normal walking. Roughening of the Co-Cr-Mo femoral heads created a 1700-fold increase in the numbers of sub-micron PE particles under fast jogging. The clinical significance of this result suggests that highly active patients will generate high numbers of bioactive PE wear particles within the accepted bioactive range, 0.2-10μm. Metal-on-metal (MOM) hip arthroplasty has also seen rapid growth worldwide. However, there remains concern over their long-term biocompatibility due to systemic ion release. Therefore, the aim of this current investigation was to test the hypothesis that larger diameter MOM bearings (greater than 40 mm) will generate smaller Co-Cr-Mo wear particles compared to a 28 mm size bearing, and reduce the total wear particle surface area, and to test the hypothesis that `severe' gait conditions will greatly increase the size of Co- Cr-Mo wear particles, thereby causing a sizable increase in wear particle surface area. Walking with a 28 mm bearing produced the largest wear rate of at 0.92 mm3/106 cycles, whereas the 40 mm and 56 mm bearings, generated lower wear rates of 0.39 mm3/106 cycles and 0.32 mm3/106 cycles respectively. Simulated fast jogging created a 3-fold increase in the number of elongated (needle) wear particles compared to normal walking, and generated a 20-fold increase in total wear particle surface area per year of use compared to normal walking. The clinical significance of this result suggests that highly active patients with MoM implants will exhibit greater ion release, although this may be minimised by using larger diameter bearings for active or younger patients.