 Indolamin 2-3-dioxygenase, IDO, is an enzyme that catalyzes the oxidative degradation of the essential amino acid tryptophan into various metabolites, including chineuronine. These metabolites have been shown to play a role in immunosuppressive and anti-inflammatory processes, making IDO an attractive target for therapeutics. This review summarizes the current understanding of the role of IDO in immune regulation, with particular emphasis on its potential use in treating autoimmune diseases, allergies, and transplant rejections. This article was authored by Jacques Simbong, Dakina A. Nicholas, Timothy W. Torres, and others.