Over the past few years regulatory T cells have been identified as key players in ensuring that the immune system does not attack self tissues and does not overreact to foreign antigens. The frequency and functional activity of these cells have been proposed markers for evaluating autoimmune status and possibly inflammatory conditions in patients. Based on studies in animal models showing that regulatory T cells when isolated and expanded can prevent organ and tissue destruction, clinical trials using these cells have been initiated. This session will focus on recent developments in our understanding of how these unique cells regulate immune responses in the setting of blood transfusions and in transplantation, discuss recent advances in our understanding of the conditions to expand these cells ex vivo, and how these findings are being translated in clinical trials.