The critical and increasing shortage of human organs for transplantation has stimulated much research into the use of pig organs and cells in humans. Previous clinical experience with xenotransplantation is briefly reviewed. Recent progress in overcoming the immunological and microbiological barriers is described, and the outstanding problems that remain in these areas and in potential physiological incompatibilities are highlighted. Consideration is given to the ethical, legal, regulatory, and economic aspects of clinical xenotransplantation, and also to the choice of the initial patients for early clinical trials. Finally, brief mention is made of potential alternatives to xenotransplantation, such as artificial organs, stem cell technology, and tissue engineering.