Allogeneic HSCT is the most reliable, robust, and effective cell-based biotherapy currently available to pediatric and adult patients with hematologic malignancies. The central role of donor-derived lymphocytes in mediating an effective antitumor effect, preventing and controlling opportunistic infections, and causing GVHD is well documented in animal experiments and human trials. The profound lymphopenia after conditioning regimens coupled with molecular tools to distinguish host versus donor cells provides investigators a window into immune recovery after allogeneic HSCT. Serial analyses of T cell subsets linking immunophenotype with function have revealed the kinetics of donor-derived T cell recovery after allografting and provided insights into ways the immune system can be manipulated to augment the graft-versus-tumor (GVT) effect without inducing GVHD. As this review demonstrates, investigators are not limited to being passive observers of this immune reconstitution; rather, we have an opportunity to shape the allografted T cells repertoire to selectively augment immune function.