Vanadium is used by microorganisms as an electron acceptor in respiration, and as an essential transition metal in enzymatic reactions. An example for the employment in respiratory function is the soil bacterium Shewanella, which reduces vanadate(V) to oxidovanadium(IV). Examples for enzymatic reactions are the nitrogen fixation (by the proteobacterium Azotobacter and the cyanobacterium Anabaeno), and the two-electron oxidation of halide X(-) to a species {[}X(+)] by marine macro-algae, fungi and lichen. In vanadium nitrogenase, vanadium is constituent of a {[}Fe(7)VS(9)] cluster, in vanadate-dependent haloperoxidases it is present in the form of H(2)VO(4)(-) bound to a histidyl residue of the protein matrix. Mushrooms of the genus Amanita store vanadium in the form of amavadin, a ``bare{''} (non-oxo) vanadium(IV) complex. Several sea squirts and fan worms accumulate vanadium from sea water and store it as an aqua complex of vanadium(III). ``Tailored{''} vanadium complexes with organic ligands have been shown to be active as insulin-mimics in vivo and in vitro: They are able to stimulate the cellular uptake of glucose and to inhibit the degradation of lipids. These functions are related to the phosphate-vanadate antagonism.