The immune repertoire of T lymphocytes invading human allografts is of fundamental importance both at the operational level, in order to achieve relevant matching, and at the functional level, since the unique capacity of T and B cells to specifically recognize allogeneic components restricts the origin of the signals leading to rejection by these cells. In this paper, the authors review their own work, as well as other contributions in this domain, with special reference to the frequency and function of donor-committed cells among the infiltrate and the relationship between T-cell receptor gene rearrangements and repertoire.