The essential role of iron for the human organism was recognised in antiquity, whence legend (and a recent review1) records that a glass of wine and rust was sufficient to cure sexual impotence and to benefit those who, unfortunately, could not ‘cohabit properly’. A complete understanding of these remarkable phenomena still escapes contemporary biochemistry, and even bioinorganic chemistry, yet, of all the essential trace metals, iron and its biological functions are probably the best understood. Many of these functions depend on the aqueous chemistry of iron(III), which is now known to be dominated by hydrolysis and polymerisation, with the formation of polynuclear iron(III) species, that is species that contain at least two iron atoms interacting via some bonding arrangement. This review discusses the results of structural studies, using a variety of physical methods, on a number of proteins that bind a large number (≥50) of iron(III) ions in a polynuclear fashion. Many of these studies have been carried out since 1969, when this research field was the subject of an excellent review1.