The MHC is highly polymorphic in most vertebrates and the suggested selective mechanisms responsible for the maintenance of this variation are several, including maternal-fetal interaction, parasite resistance, and negative-assortative mating. Evidence for these mechanisms is reviewed and estimates of the amount of selection in a number of studies are given. Although there is much yet to be understood about the mechanism and extent of balancing selection at MHC, new advances in molecular genetic technology and increasing interest in MHC from many types of biologists promise answers in the near future.