T-cell alloreactivity is a well-established phenomenon, but its molecular basis has remained enigmatic. Although there are differences between T-cell recognition of conventional and allogeneic antigens, it has become increasingly clear that they share many similarities. Recent insights into the specificity of the T-cell receptor (TCR) for peptide and the seeming intrinsic affinity of the TCR for the surface of the MHC molecule have provided a better understanding of how the TCR and peptide–MHC complexes interact. Here, we highlight the similarities and differences between conventional and allogeneic recognition of TCR–peptide–MHC complexes, and discuss how our view of allorecognitionhas changed, as well as the implications for TCR specificity and T-cell development.