Mitochondria use multiple carbon fuels to produce ATP and metabolites, including pyruvate, which is generated from glycolysis; amino acids such as glutamine; and fatty acids. These carbon fuels feed into the TCA cycle in the mitochondrial matrix to generate the reducing equivalents NADH and FADH2, which deliver their electrons to the electron transport chain. Mitochondria are complex organelles that play an important role in many facets of cellular function, from metabolism to immune regulation and cell death. Mitochondria are actively involved in a wide variety of cellular processes and molecular interactions, such as calcium buffering, lipid flux, and intracellular signaling. It is increasingly recognized that mitochondrial dysfunction is a hallmark of many diseases such as obesity/diabetes, cancer, cardiovascular and neurodegenerative diseases. Mitochondrial metabolism is a key determinant of tumor progression by impacting on functions such as epithelial-to-mesenchymal transition. Mitochondrial metabolism and derived oncometabolites shape the epigenetic landscape to alter aggressiveness features of cancer cells. Changes in mitochondrial metabolism are relevant for the survival of tumors in response to therapy.