Metabolic reprogramming of cancer cells is a phenotypic trait necessary to promote proliferation and survival. Despite past controversies, recent transcriptomic, proteomic, functional and structural studies of mitochondria of the cancer cell indicate that an impaired biogenesis and activity of the organelle is required for the development of some tumors. Cancer aggressiveness can be estimated by its bioenergetic signature, a protein ratio that correlates the expression of b-F1-ATPase of oxidative phosphorylation relative to the glycolytic GAPDH. The bioenergetic signature also provides a gauge that informs of the metabolic activity of tumors and cancer cells as well as of the response to chemotherapy. The convergence of different epithelial tumors on the same bioenergetic signature supports that it provides an important tool and common target for cancer therapy. We stress that targeting the energetic metabolism of tumors affords a valuable strategy to combat the disease.