Siderophores, defined as iron(III) specific transport compounds, are widely distributed in aerobic and facultative anaerobic microbial species. The list of microbes known to form siderophores includes various enteric bacteria; human, animal and plant pathogenic bacteria and fungi; soil microorganisms; Gram positive and negative species, blue green alga (Cyanobacteria) and higher algae; nitrogen fixing bacteria; and many types of fungi including all species of Penicillia, Neurospora, basidiomycetes and certain types of yeast. As natural products, siderophores are classed conveniently as hydroxamates, catechols and “miscellaneous”, the latter possibly structurally related to the phytosiderophores of plants. A variety of standard methods has evolved for detection, enhanced production, isolation, characterization and synthesis of the siderophores. The cloning of the enterobactin and aerobactin systems of Escherichia coli has been reported. A very large number of siderophores remain to be characterized as either known compounds or new products. The opportunities for technical exloitation of the substantial reservoir of basic research knowledge of siderophores abound in fields as diverse as clinical medicine and plant pathology.