Summary: Endowed with the ability to actively suppress an immune response, regulatory T cells (Tregs) hold the promise of halting ongoing pathogenic autoimmunity and restoring self-tolerance in patients suffering from autoimmune diseases. Through many in vitro and in vivo studies, we have learned that Tregs can function in the lymph nodes as well as in the peripheral tissues. In vivo, Tregs act through dendritic cells to limit autoreactive T-cell activation, thus preventing their differentiation and acquisition of effector functions. By limiting the supply of activated pathogenic cells, Tregs prevent or slow down the progression of autoimmune diseases. However, this protective mechanism appears insufficient in autoimmune individuals, likely because of a shortage of Tregs cells and/or the development and accumulation of Treg-resistant pathogenic T cells over the long disease course. Thus, restoration of self-tolerance in these patients will likely require purging of pathogenic T cells along with infusion of Tregs with increased ability to control ongoing tissue injury. In this review, we highlight advances in dissecting Treg function in vivo in autoimmune settings and summarize multiple studies that have overcome the limitations of the low abundance of Tregs and their hypoproliferative phenotype to develop Treg-based therapies.