There has been great attention on indoleamine-2,3-dioxygenase 1 (IDO1) around cancer immunotherapy because of its role in enabling cancers to evade the immune system. The most recent spurt of high potent IDO1 inhibitors has been driven by the solution of the increased crystal structures of inhibitors with IDO1. Though the structural information of the active site of IDO1 obtained from the crystals are quite similar, the structures of reported potent inhibitors are quite different. Besides, while thousands of bioactive small molecule inhibitors of IDO1 exist, to date, only five compounds have entered clinical trials. In an effort to obtain more clinical drugs targeting IDO1, more comprehensive understanding of the active site of IDO1 and the structures of existing potent IDO1 inhibitors are necessary. Thus, this review mainly focus on the key features reported from specific crystal structures of IDO1 and an overview of the most recently developed IDO1 inhibitors under investigation and their other derived applications which may contribute to a better usage in cancer immunotherapy.