Osteonecrosis of the jaw (ONJ) is a severe bone disease that affects the maxilla and the mandible. Various forms of ONJ have been described over the last 160 years, and a number of causes have been suggested in the literature. Osteonecrosis of the jaws associated with bisphosphonate therapy, required by some cancer treatment regimens, has been identified and defined as a pathological entity since 2003. The possible risk from lower oral doses of bisphosphonates, taken by patients to prevent or treat osteoporosis, remains uncertain.