A cardinal feature of the adaptive immune response is its ability to generate long‐lived populations of memory T lymphocytes. Memory T cells are specific to the antigen encountered during the primary immune response and react rapidly and vigorously upon re‐encounter with the same antigen. Memory T cells that recognize microbial antigens provide the organism with long‐lasting protection against potentially fatal infections. On the other hand, memory T cells that recognize donor alloantigens can jeopardize the survival of life‐saving organ transplants. We review here the immunobiology of memory T cells and describe their role in the rejection of solid organ allografts.