There is an acute shortage of human organs available for transplantation. Transplanting animal organs or tissues is a potential solution and could also provide a novel means of gene delivery. Xenotransplantation faces many challenges, including the immune response of the recipient against the transplant, the physiological limitations of the transplant and the possibility of introducing infectious organisms into the recipient. A number of medical centres around the world are addressing these issues. Much of this research focuses on the application of genetic therapies because of the unique opportunity this represents for genetic engineering to be used at the source of organ tissue, rather than to the recipient.