Osteonecrosis of the jaw (ONJ) is now defined by the presence of exposed bone in the mouth, which fails to heal after appropriate intervention over a period of six or eight weeks. It is commonly precipitated by a tooth extraction in patients treated with zoledronate, pamidronate or a combination of these agents, for the management of myeloma, breast cancer or prostate cancer. In patients with these malignancies who are treated with bisphosphonates, the overall prevalence is about 5%. There is a need to clearly delineate the incidence of ONJ in osteoporosis patients treated with bisphosphonates, and in appropriate control populations. Based on current evidence, the risk of ONJ in osteoporosis appears to be comparable to that in the general population. It is likely that ONJ results from direct toxicity to cells of bone and soft tissue from high potency bisphosphonates, probably acting through their effects on the mevalonate pathway. The bone in ONJ lesions does not appear to be ‘frozen’, rather there is very active resorption present, probably stimulated by local infection. This is likely to result in the local release at high concentrations of bisphosphonates. Management focuses on prevention, treatment of infection and cessation of bisphosphonates. The role of surgery is unclear.