The energy transduction in mitochondria, with its principal agent ATP, still represents a major challenge for biological research. In general, the energy transduction process is divided into three sections: (1) the redox processes; (2) a conservation of intermediary energy forms; (3) synthesis of ATP. All three processes are linked to the membrane and are, therefore, as difficult to resolve as are processes linked to other biomembranes. It is probable that the electron transport system is constructed in such a way as to provide energy for synthesis of ATP and related processes. Important for this function is the transversal distribution of these components across the membrane, facilitating generation of membrane potential by electron or proton transfer. The exact composition of the respiratory chain is not yet known, in particular with respect to iron-sulphur proteins. Progress is achieved by defining single species of the respiratory chain, subunit composition, amino acid sequences and genetic derivation from intra- or extra-mitochondrial translation. Energy generated by oxidation can be trapped before ATP is formed by a number of reactions, in particular reversed electron transport, energy-dependent transhydrogenation and uptake of anions or cations into the mitochondria. The latter reaction is of major importance for understanding the intermediate energy form, as it appears to use energy most directly and be driven mainly by membrane potential or proton gradient across the membrane. The formation of ATP is a major problem hindering elucidation of the mechanism of oxidative phosphorylation. The mechanism of this enzymic process is not yet understood although the enzymes have been isolated and the subunits have been defined. Most probably, a concerted reaction between ADP and phosphate, driven by some conformational transition of the complex, leads to the formation of ATP. Release of ATP from a hydrophobic to hydrophilic environment may consume most of the energy.