Although most CD4+CD25+ regulatory T (Treg) cells develop in the thymus (i.e., natural Treg or nTreg), accumulating evidence suggests that they can also develop in the periphery (adaptive/induced Treg or iTreg). Both types of cells are functionally associated with the expression of Foxp3, a transcription factor that is constitutively expressed in nTreg cells and inducible during iTreg cell generation from CD4+CD25 – T lymphocytes. Multiple factors are involved in the generation and function of Treg cells, but a major role seems to be played by indoleamine 2,3-dioxygenase (IDO). IDO can both deplete tryptophan in local tissue microenvironments and generate immunoregulatory catabolites, known as kynurenines. Tryptophan starvation and presence of kynurenines can induce the conversion of naïve CD4+CD25 – T cells into highly suppressive CD4+CD25+Foxp3+ Treg cells. In turn, Treg cells induce IDO in dendritic cells (DCs) and convert inflammatory into regulatory DCs, which can further expand the Treg cell compartment by tryptophan catabolism. Evidence suggests that the modulation of IDO activity favors the interconversion between Treg cells and T helper type 17 (TH17) inflammatory cells. Thus, in the periphery, tolerogenic immune responses mediated by Treg cells can be induced and amplified by IDO, a tryptophan catabolizing enzyme that also contributes to the plasticity of the Treg cell lineage.