Indoleamine 2,3-dioxygenase (IDO) is an intracellular heme-containing enzyme that catalyzes the initial rate-limiting step in tryptophan degradation along the kynurenine pathway. Recent works have demonstrated a crucial role for IDO in the induction of immune tolerance during infections, pregnancy, transplantation, autoimmunity, and neoplasias. IDO is widely expressed in human tissues and cell subsets, including dendritic cells, where it modulates their function by increasing tolerogenic capacities. The aim of the present paper is to highlight the most recent data about IDO expression in dendritic cells and its role as a potent inducer of T regulatory cells.