The major histocompatibility complex is a group of closely linked loci that code for molecules used by T-lymphocytes as context for the recognition of antigens. The loci fall into two classes: I, coding for molecules used as context by cytotoxic T-lymphocytes and II, used as context by helper and other regulatory T-cells. The Mhc is present in all mammals and perhaps all vertebrates. Some of the Mhc loci are highly polymorphic, while others are not. This article will summarize what is known about the genetic organization of the Mhc in different species and will discuss the selection pressures acting on the individual loci and the tempo and the mechanisms of their evolution.