Tomorrow: The future of xenotransplantation is still uncertain—technical difficulties and the possibility of accidentally introducing animal pathogens into people may yet prove to be show-stoppers. But one thing seems clear. As society debates the ethics of transplanting animal tissues into humans, scientists will probe the secrets of the human immune system. And their discoveries will undoubtedly boost the success of transplantation, whether the organs come from a caring relative or an engineered pig.