Most reducing equivalents extracted from foodstuffs during oxidative metabolism are fed into the respiratory chains of aerobic bacteria and mitochondria by NADH:quinone oxidoreductases. Three families of enzymes can perform this task and differ remarkably in their complexity and role in energy conversion. Alternative or NDH-2-type NADH dehydrogenases are simple one subunit flavoenzymes that completely dissipate the redox energy of the NADH/quinone couple. Sodium-pumping NADH dehydrogenases (Nqr) that are only found in procaryotes contain several flavins and are integral membrane protein complexes composed of six different subunits. Proton-pumping NADH dehydrogenases (NDH-1 or complex I) are highly complicated membrane protein complexes, composed of up to 45 different subunits, that are found in bacteria and mitochondria. This review gives an overview of the origin, structural and functional properties and physiological significance of these three types of NADH dehydrogenase.