Hydroxyapatite-coated implants have demonstrated extensive bone apposition in animal models. The osseous interface develops even in the presence of gaps of 1 mm and relative motion of up to 500 mum. Development of implant-bone interfacial strength is due to the biological effects of released calcium and phosphate ions, although surface roughness leads to increased interface strength in the absence of interface gaps. The clinical results at fifteen years after total hip replacements have demonstrated that hydroxyapatite-coated femoral stems perform as well as, and possibly better than, other types of cementless devices, with the added benefit of providing a seal against wear debris. Hydroxyapatite-coated acetabular components must have a mechanical interlock with bone in order to take advantage of the coating effects. Clinical analyses of these types of designs at seven years have indicated good survivorship. The performance of a hydroxyapatite-coated implant depends on coating properties (thickness, porosity, hydroxyapatite content, and crystallinity), implant roughness, and overall design. The most reliable predictor of the performance of a device is success in long-term clinical studies.