Purpose of reviewRecent developments toward the successful clinical application of autologous and allogeneic mesenchymal stem cells (MSCs) to organ transplantation are summarized with a focus on safety and efficacy.Recent findingsClinical trials in organ transplantation and other conditions indicate that infusion of autologous or allogeneic MSCs is generally well tolerated. However, new studies also suggest that efficacy may be curtailed by sequestration in the lungs and early elimination. Safety concerns regarding autologous and/or allogeneic MSCs that have recently been investigated include transient proinflammatory effects, influences on opportunistic infections and cancers and alloantibody induction. Animal models indicate that autologous MSCs are likely to be efficacious in preventing or treating early intragraft inflammation and may reduce the risk of acute rejection - observations that have been borne out in a randomized controlled trial of living-donor kidney transplantation. The potential for donor-specific or third-party allogeneic MSCs to promote allograft tolerance is suggested by animal model studies but has not yet been proven in humans.Summary: Recent reports on the safety and efficacy of autologous MSCs for early posttransplant outcomes give cause for optimism. Benefits of allogeneic MSCs for long-term allograft survival and of MSCs for chronic transplant injury await clinical validation.