Iliac crest bone graft has long been the standard adjunct used in spine fusion surgery. This graft provides osteogenic, osteoinductive, and osteoconductive elements that aid in creation of a fusion mass. However, morbidity associated with bone graft harvest has led surgeons to seek other potential adjuncts, including bone morphogenetic proteins, demineralized bone matrix, and graft expanders such as synthetic bone graft and allograft. Knowledge of fusion biology is required to understand the benefits and limitations of these agents, which promote fusion via one of four mechanisms: osteogenesis, osteoinduction, osteoconduction, and osteopromotion. Although bone morphogenetic proteins have shown a clear ability to aid in bone formation and successful fusion, recent concern regarding their safety has tempered enthusiasm regarding their use.