The xenotransplantation research is driven by the increasing gap between the number of patients with end-stage organ failure on waiting lists for transplantation and the supply of allografts. The lack of success in developing suitable artificial organs for permanent treatment of organ failure has further strengthened the need for xenotransplantation research. Pigs are now generally accepted to be the source animal of choice. Transplantation of pig organs to humans faces several barriers which have to be overcome before it comes to clinical application: (1) anatomical and physiological conditions; (2) immunological rejection mechanisms; (3) molecular compatibility between signal molecules of the two species; (4) risk of transmission of microorganisms, particularly pig endogenous retroviruses; and (5) legal and ethical aspects both with respect to the animal and the recipient. Here we will focus on the role of the complement system in the rejection of immediately vascularized pig-to-primate xenografts. The hyperacute rejection occurring within minutes after transplantation is mediated by binding of natural antibodies to the Galalpha(l-3)Gal epitope on the endothelial cells with subsequent complement activation. Whereas inhibition of complement activation protects against hyperacute rejection, the role of complement in the later rejection phases is less clarified.