Pyoverdine, the yellow-green, water-soluble, fluorescent pigment of the fluorescent Pseudomonas species, is a powerful iron(III) scavenger and an efficient iron(III) transporter. As a fluorescent pigment, it represents a ready marker for bacterial differentiation and, as a siderophore, it plays an important physiological function in satisfying the absolute iron requirement of these strictly aerobic bacteria. Close to 40 structurally different pyoverdines have been identified to date, each characterized by a different peptidic part of the molecule and by a very narrow specificity as an iron transporter for Pseudomonas species, usually restricted to the producer strain or to strains producing an identical compound. Cross-reactivity does occur, however, for pyoverdines exhibiting partial identity at the peptide chain level, suggesting some information on the receptor-recognition site of the molecule. With the recent description of an operonic cluster of four genes involved in the synthesis of the chromophoric part of the molecule, a total of seven pyoverdine biosynthetic genes have been identified so far in Pseudomonas aeruginosa PAO1. Although the precise function of the gene products needs further clarification, a biosynthetic pathway based on a multienzyme thiotemplate mechanism allowing a step-by-step synthesis of the whole chromopeptide molecule can be postulated. A promising future is expected from recent developments which indicate that pyoverdines might be considered as potent and easy-to-handle taxonomic markers for the fluorescent species of the genus Pseudomonas.