There are two major hurdles on the way of xenotransplantation entering the clinic. The first is the technical issue of being able to overcome the human immune response that leads to rejection of transplanted organs from other species. The second concerns the potential risk of inadvertent transfer of animal viruses such as the much discussed porcine endogenous retroviruses (PERV) which might cross the species barrier and cause disease in human hosts. Many closely monitored clinical trials are needed as an approach to assess the safety and efficacy of using porcine cells, tissues, or organs therapeutically in humans.