Presently provided are IDO inhibitors and pharmaceutical compositions thereof, useful for modulating an activity of indoleamine 2,3-dioxygenase; treating indoleamine 2,3-dioxygenase (IDO) mediated immunosuppression; treating a medical conditions that benefit from the inhibition of enzymatic activity of indoleamine-2,3-dioxygenase; enhancing the effectiveness of an anti-cancer treatment comprising administering an anti-cancer agent; treating tumor-specific immunosuppression associated with cancer; and treating immunosuppression associated with an infectious disease.