Having been a taboo subject for many years, dedicated regulatory T cells are now center stage. To the transplantation immunologist, this should not come as a surprise, given that transferable T-cell tolerance has remained a robust phenomenon over the past 2 decades. Recent insights into the cell populations mediating this regulation and, to a limited extent, into their mechanisms of action have heightened interest in the possibility of deliberately inducing such cells with specificity for donor alloantigens, as a means of promoting transplantation tolerance. This review discusses the range of regulatory T cells that have been reported, and the complementary roles of deletion and regulation in the development of tolerance to transplanted tissues.