This text provides summaries of the main systems of blood group antigens and MHC molecules. The human systems are used to explain the necessary background knowledge and the systems in nonhuman primates are then described and compared. The emphasis is on the molecular nature of the gene products, the evolutionary relationships among the various systems, and the presumed mechanism by which the systems have evolved. All articles are written in such a way that they can serve as an introduction to the field for nonexperts, and at the same time as a handbook for experts. Numerous tables, diagrams and figures provide overviews of structure, distribution in different species, lists of known forms, and evolutionary relationships.