In the thymus, a diverse and polymorphic T-cell repertoire is generated by random recombination of discrete T-cell receptor (TCR)-αβ gene segments. This repertoire is then shaped by intrathymic selection events to generate a peripheral T-cell pool of self-MHC restricted, non-autoaggressive T cells. It has long been postulated that some optimal level of TCR diversity allows efficient protection against pathogens. This article focuses on several recent advances that address the required diversity for the generation of an optimal immune response.