Osteolysis and aseptic loosening are the major causes of failure of total hip replacements (THRs). These processes are induced by wear debris released from components of implants over time. Wear particles are a clinically relevant size (0.1-10 microm) that activates macrophages and induces a foreign-body response. Activated macrophages stimulate the production of proinflammatory cytokines (IL-1, IL-6, and TNF-alpha) to the synovial capsule, leading to chronic inflammation and bone resorption. Wear mechanics, including characteristics of wear particles from metal, polyethylene, and ceramic articulating surfaces, fixation materials, and surface coatings, have been reported in the literature and are compiled here. Data concerning the pathophysiological effects of wear debris are reported in this article, whereas data on the metal ions released from implants are presented in the following article (see pp. 113-114). Clinical data obtained from in vivo use and laboratory studies conducted in vitro are summarized.