Allograft bone continues to play an important role in revision hip and knee arthroplasty. A basic understanding of allograft biology and immunology is important in order to increase the success of allografting. Although the literature has a wealth of knowledge on the subject there are still many unknowns. The role of immunology in bone transplantation has been known for a long time, but only recently has it become apparent that the bone remodeling system and the immunologic system interact to affect the clinical success of bone transplantation. Neither of these two systems are completely understood nor is their interaction. Future research in the field of bone transplantation will be aimed at a better understanding of these systems individually but, more important, how they interact in humans. Until that time, allografting still can be used with success if one understands the role of allograft biology, immunology, and the important role of the host environment in bone transplantation.