Osteolysis and aseptic loosening are the major causes of failure of total hip replacements. Particles released from the implant during articulation have been found to be a clinically relevant size (0.1–10 μm). Wear debris particles and toxic metal ions activate macrophages and stimulate the release of pro-inflammatory mediators IL-1, IL-6, TNF-α and PGE2, which activate osteoclasts and lead to bone resorption. Data is compiled to reflect trends in the amounts of wear debris produced by different articulating surfaces. This article overviews the results of clinical studies, in vivo, and laboratory studies, in vitro, and provides a critical discussion of results.