The transition element molybdenum is of essential importance for (nearly) all biological systems. It needs to be complexed by a special cofactor in order to gain catalytic activity. With the exception of bacterial Mo-nitrogenase, where Mo is a constituent of the FeMo-cofactor, Mo is bound to a pterin, thus forming the molybdenum cofactor Moco, which in different versions is the active compound at the catalytic site of all other Mo-containing enzymes. In eukaryotes, the most prominent Mo enzymes are nitrate reductase, sulfite oxidase, xanthine dehydrogenase, aldehyde oxidase, and the mitochondrial amidoxime reductase. The biosynthesis of Moco involves the complex interaction of six proteins and is a process of four steps, which also requires iron, ATP, and copper. After its synthesis, Moco is distributed to the apoproteins of Mo enzymes by Moco-carrier/binding proteins. A deficiency in the biosynthesis of Moco has lethal consequences for the respective organisms. In humans, Moco deficiency is a severe inherited inborn error in metabolism resulting in severe neurodegeneration in newborns and causing early childhood death. Eubacteria possess different versions of the pteridin cofactor as reflected by a large number of enzymes such as nitrate reductase, formate dehydrogenase, and dimethyl sulfoxide reductase, while in archaea a tungsten atom replaced molybdenum as catalytic metal in the active center.