Nearly a century ago, Otto Warburg found that cancer cells can shift their energy production to aerobic glycolysis instead of the more efficient oxidative phosphorylation. An emerging hallmark of many cancers is their metabolic plasticity, as rewiring of cancer cells' metabolic fluxes is essential for their proliferative and invasive agenda. With increasing understanding of these alterations, researchers are identifying features of tumor metabolism that can be used for novel diagnostic, prognostic, and therapeutic opportunities.