Understanding the nature of allorecognition is fundamental to the design of antigen-specific therapies for transplantation. As recently as 10 years ago it was generally believed that recognition of major histocompatibility complex (MHC) molecules by T lymphocytes was direct and represented the “simplest” kind of T-lymphocyte interaction. It is now clear that the nature of allorecognition is complex, involving a variety of different forms of MHC antigens with or without peptides contained in their antigen-binding groove. In addition, there is renewed interest in alternative forms of allorecognition, including so-calledindirect allorecognition, in which donor alloantigens are recognized as peptides in the context of recipient self. Lastly, it appears that cells other than T lymphocytes, eg, natural killer cells, are capable of antigen-specific recognition and may be responsible for heretofore underappreciated mechanisms of transplant rejection.