Pyrroloquinoline quinone （PQQ） is a redox cofactor identified from bacterial methanol dehydrogenase, and has been found in trace amounts in a variety of foods and in human tissues lacking PQQ biosynthesis.　It has been reported that mice fed a PQQ-deficient diet develop deficiency symptoms such as growth retardation, skin fragility, and reduced reproductive capacity, suggesting that PQQ may be a vitamin.　However, the enzyme that requires PQQ as a coenzyme in mammals has not been identified, and thus PQQ has not yet been certified as a vitamin. This paper reviews the biochemical properties, physiological functions, and mechanism of action of PQQ.