Transplantation has become a routine therapy for end-stage organ disease. Despite improvements in patient and allograft outcomes, chronic rejection and the complications from lifelong immunosuppressive therapy remain significant problems. The induction of transplantation tolerance, indefinite allograft acceptance independent of chronic immunosuppressive therapy, remains the ultimate ambition in transplantation. Recent advances in immunobiology have described costimulatory pathways that are critical for T-cell function. Numerous tolerance strategies have incorporated agents designed to manipulate these pathways. Although new molecules continue to be described, their use alone has failed to promote lasting tolerance. The use of costimulation blockade in nonmyelosuppressive mixed chimerism strategies has emerged as one of the most promising strategies thus far.