Introduction: The inhibition of indoleamine 2,3-dioxygenase (IDO) has emerged as a key area in cancer immunotherapy in the past decade. Despite the large variety of potential inhibitors screened so far, the number of active scaffolds remains limited. Areas covered: All relevant patent literature published between 2008 and 2012 is reviewed. Representative examples are given for each patent and/or class of compounds along with data (if available) on their inhibitory activity. The presentation is deepened by additional data published in peer-reviewed literature. Expert opinion: Key events that stimulated the search of IDO inhibitors are presented. To date, however, the number of available scaffolds remains limited with only one confirmed inhibitor (from Incyte Corp.) in the clinic. Major challenges in the search for IDO inhibitors are discussed as well as the relevance of selectivity of IDO inhibition versus inhibition of tryptophan 2,3-dioxygenase. © 2013 Informa UK, Ltd.