Indoleamine 2,3-dioxygenase (IDO) catalyzes the first and rate-limiting step of Kynurenine pathway along the major route of Tryptophan catabolism. The scientific interest in the enzyme has been growing since the observations of the involvement of IDO in the mechanisms of immune tolerance and in the mechanisms of tumor immuno-editing process. In view of this latter observation, in particular, preclinical studies of small molecule inhibitors of the enzyme have indicated the feasibility to thwart the immuno-editing process and to enhance the efficacy of current chemotherapeutic agents, supporting the notion that IDO is a novel target in cancer disease.This review covers the structural and conformational aspects of substrate recognition by IDO, including the catalytic mechanism and the so-far puzzling mechanisms of enzyme activation. Furthermore, we discuss the recent advances of medicinal chemistry in the field of IDO inhibitors.