The most common source of bone graft used to be autograft, the transfer of a small piece of the patient's own bone from one site, usually the iliac crest, to another. However, the amount that can be harvested safely is limited; it increases surgery time and may be complicated by donor site morbidity. The use of allograft, supplying bone graft from other patients via bone banks, overcomes these difficulties. However, the demand for allograft now exceeds the supply, there is a small but finite risk of disease transmission, and it is highly variable in strength. These factors have led to increasing interest in the use of synthetic bone graft substitutes.