The proton-pumping NADH:ubiquinone oxidoreductase is the foremost of the respiratory chain complexes providing the proton motive force required for the synthesis of ATP. The complex is found in purple bacteria and in the mitochondria of most eukaryotes. The bacterial complex consists of 14 different subunits while the mitochondrial complex contains at least 28 accessory proteins which do not directly participate in the electron and proton transport function. A homologous complex which has 11 subunits in common with the respiratory complex I exists in cyanobacteria and chloroplasts. This complex might probably work as a NADPH:plastoquinone oxidoreductase being possibly involved in a cyclic photosynthetic electron transport. Homologues of the functional modules of the complex are also found in other bacterial electron transfer and ion transport proteins. The modular evolution of the complex and the possible origin of its modules are discussed in this paper.