The recent crystal structure determinations of MHC class I molecules with single bound peptides have allowed us to understand the guidelines that govern peptide binding in a given MHC allele. Evolution has provided for MHC class I molecules whose antigen binding clefts possess distinct physical and chemical properties as a result of the particular arrangement of variable residues lining the binding cleft. As a result, a given molecule binds a unique set of peptides, the position and identity of whose anchor residues are dictated by the features of the cleft. In addition to the interactions between the anchor residues and the cleft, the peptide is held in the groove by a highly conserved array of hydrogen bonds. A tantalizing application of our newfound understanding of peptide binding will be in the design of model peptides that either block or enhance immune response, in order to achieve effective treatments for a variety of disorders of the immune system.