Peripheral tolerance is mediated by multiple mechanisms such as anergy and/or active suppression of effector T cells by T regulatory (Tr) cells. Among the CD4⁺ Tr cells, T regulatory type 1 cells (Tr1) have been shown to down-modulate immune responses through production of the immunosuppressive cytokines IL-10 and TGF-β. Tr1 cells maintain peripheral tolerance, control autoimmunity, and prevent allograft rejection and graft versus host disease (GvHD). Cellular therapy with ex vivo generated Tr1 cells has been proven to be effective in several preclinical models of T cell-mediated pathologies and therefore, represents a promising approach for clinical application. This review will summarize the new findings on Tr1 cells, the recent development of methods for their ex vivo expansion, and their potential clinical relevance as cellular therapy.