Cells require metal ions as cofactors for the assembly of metalloproteins. Principally one has to distinguish between metal ions that are directly incorporated into their cognate sites on proteins and those metal ions that have to become part of prosthetic groups, cofactors or complexes prior to insertion of theses moieties into target proteins. Molybdenum is only active as part of the molybdenum cofactor, iron can be part of diverse Fe-S clusters or of the heme group, while copper ions are directly delivered to their targets. We will focus in greater detail on molybdenum metabolism because molybdenum metabolism is a good example for demonstrating the role and the network of metals in metabolism: each of the three steps in the pathway of molybdenum cofactor formation depends on a different metal (iron, copper, molybdenum) and also the enzymes finally harbouring the molybdenum cofactor need additional metal-containing groups to function (iron sulfur-clusters, heme-iron).