Compounds of formula (I), and pharmaceutically acceptable salts thereof, in which each compound is adapted to occupy the binding site of human IDO, which comprises a large hydrophobic pocket A and a second, proximal hydrophobic pocket B, the compound comprising at least one of the following elements: (i) a large hydrophobic fragment to substantially fill pocket A in the binding site of human IDO; (ii) an atom that can coordinate to the heme iron of human IDO, (iii) a positively charged group that can form a salt-bridge with the heme 7-propionate of the human IDO; (iv) a negatively charged group that can form a salt-bridge with Arg231 of the human IDO; (v) a hydrophobic group that can form van der Waals interactions with pocket B; and (vi) one or more substituents that can hydrogen bond to Ser167 and to Gly262, and as IDO inhibitors and their therapeutic use, eg in the treatment of cancer.