L-Tryptophan (Trp) and its metabolites have critical roles in different physiological and immunological processes. Trp is a constructive building unit of proteins to the immune system responses. Indoleamine 2, 3-dioxygenase (IDO) is a rate-limiting enzyme that metabolizes Trp into kynurenine (Kyn). Tumor cells and cancer-associated cells release IDO into the tumor microenvironment (TME). These cells include dendritic cells (DCs), endothelial cells, tumor-associated macrophages (TAMs), cancer-associated fibroblasts (CAFs), mesenchymal stromal cells (MSCs), and myeloid-derived suppressor cells (MDSCs). Additionally, IDO has a significant effect on immune responses against tumor cells. Besides inhibiting the immune system, IDO also contributes to cancer development via promoting angiogenesis, interacting with inhibitory immune checkpoints, and modulating gut microbiota. IDO causes tumor cell growth and metastasis. Thus, because IDO inhibits the tumor-specific immune responses, it has been considered an appropriate target. This chapter highlights the effects of IDO on the function of immune cells in the context of empirical cancer studies. Furthermore, we will discuss IDO as a target in tumor immunotherapy.