Mixed hematopoietic chimerism is an attractive approach for use in clinical organ transplantation, as it leads to lifelong and robust donor-specific tolerance. Despite numerous successful experimental models, however, routine clinical application has so far been precluded, mainly by the toxicity associated with the conditioning necessary to achieve allogeneic hematopoietic stem cell engraftment. Substantial progress has been achieved in allogeneic and xenogeneic models, notably including the development of considerably less toxic conditioning regimens, and insights into the mechanism of tolerance induction through mixed chimerism have been obtained.