Indoleamine 2,3-dioxygenase (IDO, also referred to as IDO1) has been demonstrated to be a normal endogenous mechanism of acquired peripheral immune tolerance in vivo. This manuscript reviews the currently available data on the role of IDO in cancer and the current results obtained with IDO inhibition in clinical trials in humans. Preliminary results with IDO inhibitors, usually combined with other anti-cancer drugs, seem encouraging. Further studies are needed to clarify the conditions in which IDO inhibitors can be of value as an anti-cancer strategy. In addition, further research should address whether the expression of IDO in tissue or blood can be a marker to select patients who can benefit most from IDO inhibition.