Organ transplantation has encountered great development during the 80's. However, the number of organ donations and transplantations performed stabilized during the 90ies, with a concomitant increase of patients on the waiting list. Xenotransplantation, i.e. the use of animal organs for transplantation to humans, is one among various alternatives to human organ donation. Xenotransplantation offers several advantages, e.g. it would be possible to transplant all patients at an early stage of their disease. The main barriers to xenotransplantation are the strong immunological responses that human develop against animal antigens and zoonoses. To overcome these hurdles, genetically modified pigs have been engineered by cloning and could allow the initiation of new clinical trials in a near future.