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Hip resurfacing arthroplasty (HRA) has a long clinical history and has utilized a variety of bearing material combinations with mixed clinical success. This paper reviews the local and systemic consequences of metal-on-metal hip resurfacing implants. Local tissue reactions cover a spectrum from no clinically adverse effects to adverse complications leading to revision associated with pain, osteolysis, or pseudotumor formation. Factors affecting the local tissue response to any implant include surgical, implant and patient factors. Metal particle size, shape and volume are important to the response around metal-on-metal implants. Surgical placement, particularly of the acetabular component, is important to the generation of wear debris. The number of patients with a sensitivity to the cobalt or chromium constituents of metal-on-metal implants is thought to be low and the cause and effect between patient sensitivity and implant failure is considered to be uncertain. Systemic toxicity from chronic exposure to the ions from metal-on-metal implants is still extremely rare although studies into potential cardiac or neurologic effects have suggested that subtle changes may be present that warrant continued and larger follow-up studies of longer-term cohorts.