The main subsets of CD4+ regulatory T (Tr) cells are the CD4+ CD25+ Tr cells and the type 1 regulatory (Tr1) cells. Both subsets are essential for the maintenance of peripheral tolerance. CD4+ CD25+ Tr cells control immune homeostasis (e.g., in autoimmunity) mainly by cell contact-dependent interactions. In contrast, Tr1 cells regulate immune responses in transplantation, allergy, and autoimmune diseases, via an IL-10- and TGF-beta-dependent mechanism. Identification of the mechanisms responsible for the induction of the different Tr subsets in vivo is a matter of intense investigation. Studies on dendritic cells (DC), performed in the last years by several groups, have significantly contributed to clarify this point. Indeed, it is now clear that the role of DCs is not only to sense danger, but also to tolerize the immune system to antigens (Ags) encountered in the absence of maturation/inflammatory stimuli. Therefore, if a naïve T cell encounters the antigen on immature DCs (iDCs), it differentiates into a Tr cell rather than an effector T cell. A better understanding of the mechanisms underlying the induction and functions of Tr cells in controlling the immune system is critical in view of a future cellular therapy to modulate immune-mediated pathologies.