Advanced microsurgical techniques have allowed performing different composite tissue allotransplantations (CTA), although immunology and pharmacotherapy still have many unresolved questions. Different CTA models were developed to explore and answer these questions during the past 25 years. Most experimental transplants to date have been performed in rats, and the immune system of the rats has fundamental differences from that of the humans, such as the lack of expression of class II antigens on the endothelial cells.(7) Over the past 20 years, we have developed 17 CTA models of the rat in our laboratory in an effort to characterize differences in the mechanism of graft acceptance and rejection. These CTA models were tested to (1) determine the mechanism of CTA acceptance and rejection; (2) determine the immunologic pattern of responses of different tissue types in CTA; (3) evaluate the immunologic response of transplant grafts of different dimensions and skin components of the most antigenic of CTA tissues(10); (4) test the effects of vascularized bone marrow transplantation (VBMT) on immunotolerance; (5) develop new immunosuppressive and immunomodulatory treatment protocols; and (6) correlate histological, morphological, and functional responses of CTA with different immunosuppression protocols. We overwrite CTA models according to the following classification: vascularized bone marrow transplant models, vascularized skin allograft models, composite allograft transplantation models, and other models (Tables 51.1 and 51.2).