Oncogene-driven proliferative signaling in tumor cells requires comprehensive upregulation of cellular energy metabolism and macromolecule syntheses. These alterations are now known to include not only upregulated glycolysis, but also increased fatty acid metabolism, glutaminolysis, deregulated mitochondrial function and more. Many prospective targets for tumor-specific pharmacological modulation of metabolism have therefore been identified. While the prospective drugs do not necessarily show very high antitumor activity by themselves, they may by depriving tumor cells of energy and building blocks for repair and proliferation come to be of major clinical use as potentiators of standard chemotherapeutic drugs and/or radiation. To this end, not only inhibitors of specific enzyme functions are being investigated, but also drugs affecting the complex signaling networks that regulate organismal and cellular energy status. This review provides examples of how modulators of energy metabolism (MEMs) targetting different aspects of tumor cell metabolism have been found to potentiate cancer treatment in vitro and in vivo.