Molybdenum (Mo) is an essential trace element and is a component of vitamin and mineral supplements. Molybdenum has essential biological roles in organisms and microorganisms. Molybdenum is the only trace metal of the second row of the periodic table that exhibits biological activity when it is ligated to a cofactor. It acts as a critical cofactor in several molybdenum-dependent enzymes that are involved in important cellular reactions and pathways, including xanthine oxidoreductase. In nature two principal concepts of Mo cofactors have evolved, one is the iron Mo cofactor in bacterial nitrogenase and the other is represented by a large family of enzymes with more than 100 representatives relying on the pterin-based Mo cofactor (Moco). Moco-containing enzymes catalyze key redox reactions in the global carbon, sulfur and nitrogen cycles. Four molybdenum-dependent enzymes are known in humans, each harboring a pterin-based molybdenum cofactor (Moco) in the active site. In these enzymes, molybdenum catalyzes oxygen transfer reactions from or to substrates using water as oxygen donor or acceptor. Molybdenum shuttles between two oxidation states, Mo(IV) and Mo(VI). Following substrate reduction or oxidation, electrons are subsequently shuttled by either inter- or intra-molecular electron transfer chains involving prosthetic groups such as heme or iron-sulfur clusters. In all organisms studied so far, Moco is synthesized by a highly conserved multi-step biosynthetic pathway. A deficiency in the biosynthesis of Moco results in a pleitropic loss of all four human Mo-enzyme activities and in most cases in early childhood death. For the general population, the diet is the most important source of molybdenum and concentrations in water and air usually are negligible. The average daily dietary intake is about 0.1-0.5 mg m.o. Molybdenum is marketed both as a tablet and as a liquid supplement containing the mineral in dissolved form. Despite widespread claims, there is no evidence that one form of molybdenum is absorbed to a markedly superior extent than any other. Current marketing of molybdenum products for the treatment of medical conditions is not founded on any meaningful scientific evidence. People with serious kidney disease should avoid taking molybdenum (or any other supplement) except on the advice of a physician. People with serious kidney disease should also avoid taking molybdenum (or any other supplement) except on the advice of a physician.