The chloroplast genome of higher plants contains eleven ndh genes encoding homologs of mitochondrial complex I subunits. The enigmatic occurrence of the respiratory complex in chloroplasts suggests the possibility that NAD (P) H dehydrogenase catalyzes cyclic electron flow around photosystem I and/or chlororespiratory electron flow, as it does in cyanobacteria. Establishment of a chloroplast transformation technique in tobacco facilitated the reverse genetic approach. Although some conclusions are still matters of controversy, it is sure that the gene product, NDH, functions to donate electrons to plastoquinone. Although NDH is dispensable under mild growth conditions, ndhB disruptant is more sensitive to high light stress, suggesting that NDH functions in minor compensation for the electron flow to cope with changes in environmental conditions. Here, we review the physiological function of NDH in chloroplasts, including important subjects that are still unclear.