The Kynurenine pathway plays an essential role in the metabolism of the amino acid tryptophan. The role of tryptophan and its metabolites, including 3-hydroxykunurenine (3-HK), quinolinic acid (QA), kynurenic acid (KA), and others are receiving attention due to their immunomodulatory and neuromodulatory effects. Enzymes in the pathway include Tryptophan Hydroxylases, Kynurenine 3-Monooxygenase (KMO), Indoleamine 2,3 dioxygenases (IDO), Kynurenine Aminotransferase 1 (KAT1), and KAT2 (alpha-Aminoadipate Aminotransferase). Most are now studied for their potential as drug targets. Dysfunction or modulation of the Kynurenine pathway has been linked to a range of nervous systems disorders, including Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, schizophrenia, and others. IDO activity, specifically, exerts an immunosuppressive effect and shows potential as an immune checkpoint target for cancer immunotherapy.