Microbial metabolism involves the chemical exchange of electrons through oxidation and reduction reactions—the equivalent of an electric current. This chapter focuses on the transport of electrons between donor and acceptor molecules, through membrane-embedded complexes that generate ion gradients across the membrane. These ion gradients, usually in the form of a proton gradient known as the proton motive force, can be used to generate ATP via ATP synthase. This transport of electrons is fundamental to respiration, the oxidative completion of catabolism to CO2 and water. It also underlies the autotrophic means of building biomass: the gain of energy from light (phototrophy) and from mineral oxidation (lithotrophy).