The driving force for all life as we know it is derived from reduction-oxidation (redox) reactions. Most biological oxidations are often coupled to cellular energy production. Typically, carbon compounds (such as carbohydrates) are oxidized to carbon dioxide while oxygen is reduced to water. Enzymes play a significant role in connecting the series of redox reactions, ultimately involving oxygen. In mitochondrial electron transport chain, electrons are passed from NADH along a series of electron acceptors/donors (oxidants and reductants) to O2. Molecular oxygen is the final oxidant (terminal electron acceptor) of aerobic metabolism. Biological reductions—on the other hand—are employed to store energy in chemical forms for later use. In photosynthetic organisms, the reduction of carbon dioxide (to carbohydrates) is powered by sunlight while water is oxidized (to oxygen). This broad canvas of redox reactions serves to drive pumps, maintain concentration gradients across membranes, and generate metabolites that have high group transfer potential and/or are energy rich. Not surprisingly, oxidoreductases form a significant group (EC 1.x.x.x) of well represented enzymes (Chap. 7 ).