Human donors have not met the growing need for transplantable organs. Purpose-bred pigs may potentially be used as a source of donor organs. However, the immunologic barriers to xenotransplantation are formidable. Considerable progress has been made in overcoming immediate hyperacute rejection. However, delayed xenograft rejection occurs and responds poorly to conventional immunosuppression. Whether this process can be controlled by the use of organs derived from pigs that do not express the xenoantigen Gal α1-3 Gal is not known. Porcine cardiac and renal xenografts can sustain life for several weeks. The long-term efficacy of these grafts will need to be reassessed when delayed xenograft rejection is better controlled. Significantly less is known regarding the ability of liver and lung xenografts to provide life-supporting function. Extracorporeal pig liver perfusion can provide short-term metabolic support for patients with liver failure, and pig hepatocytes appear to correct liver failure in cirrhotic rodents. The risks associated with pig endogenous retroviruses remain unclear.