The practice of clinical and experimental transplantation continues to evolve at a rapid pace. To appreciate the current transplant practices, it is first necessary to review transplant immunology in its proper context, ie, as a component of the complex series of events that promote the repair of damaged tissues. These processes are generally catagorized as inflammation, immunity, and tissue repair/reinforcement. In general, there are 3 forms of graft rejection: hyperacute, acute, and chronic rejection. All 3 forms of graft rejection represent pathologic consequences of one or more of these repair-related processes. The various graft rejection responses also illustrate several complex immunologic principles that need to be considered. These include the definition of an alloantigen, the structure and function of major histocompatibility complex molecules, and the behavior of antigen-presenting cells and alloreactive T cells. This review combines these concepts and principles into a discussion of the 3 forms of graft rejection, each of which is addressed at the level of histopathology, pathobiology, incidence, and clinical strategies.