The discovery of somatic mutations in the isocitrate dehydrogenase (IDH) enzymes through a genome-wide mutational analysis in glioblastoma represents a milestone event in cancer biology. The nature of the heterozygous, point mutations mapping to arginine residues involved in the substrate binding inspired several research teams to investigate their impact on the biochemical activity of these enzymes. Soon, it became clear that the mutations identified impaired the ability of IDH1 and IDH2 to catalyze the conversion of isocitrate to α-ketoglutarate (αKG), whereas conferring a gain of a novel enzymatic activity leading to the reduction of αKG to the metabolite D2-hydroxyglutarate (D-2HG). Across glioma as well as several hematologic malignancies, mutations in IDH1 and IDH2 have shown prognostic value. Several hypotheses implicating the elevated levels of D-2HG and tumorigenesis, and the therapeutic potential of targeting mutant IDH enzymes will be discussed.