There are two important questions concerning the T-cell antigen receptor. Firstly, what is the structural basis of major histocompatibility complex (MHC)-restricted recognition? Secondly, what is the origin of tolerance to self-MHC and the 'learning' of MHC-restricted recognition during thymus development? These central questions are discussed here in the context of our current knowledge of the structures of the genes encoding T-cell receptor proteins, their expression during thymocyte development and the mechanisms by which the T-cell receptor repertoire is generated.