The human genome has 5 isocitrate dehydrogenase (IDH; EC 1.1.1.42) genes, coding for 3 distinct IDH enzymes whose activities are dependent on either NADP (NADP+-dependent IDH1 and IDH2) or NAD (NAD+-dependent IDH3). Both IDH2 and IDH3 are localized in the mitochondria and participate in the citric acid cycle for energy production, whereas IDH1 is localized in the cytoplasm and peroxisomes. IDH enzymes catalyze the oxidative decarboxylation of isocitrate to produce α-ketoglutarate (also known as 2-oxoglutarate) and concomitantly produce NADPH from NADP+. IDH enzymes also catalyze the reductive carboxylation of α-ketoglutarate to form isocitrate and concomitantly produce NADP+ from NADPH. Since IDH1 and IDH2 are mutated in 50%–80% of astrocytomas, oligodendrogliomas, oligoastrocytomas, and secondary glioblastomas, Isocitrate dehydrogenase (IDH) enzymes have recently become a focal point for research aimed at understanding the biology of glioma. Cancer-associated point mutations in IDH1 and IDH2 confer a neomorphic activity that allows reduction of αKG to the oncometabolite (R)-2-hydroxyglutarate (2HG). High concentrations of 2HG have been shown to inhibit αKG-dependent dioxygenases, including histone and DNA demethylases, which play a key role in regulating the epigenetic state of cells.