By definition biological iron-sulfur clusters are structures of sulfide and iron ions forming a core that is externally coordinated by a protein. As a result of many years of descriptive biochemical studies (cf. [1-5]) one can formulate a more specific operational definition in the form of a list of characteristics for which until now no counter-examples have been found. The irons are always high-spin Fe(III) or high-spin Fe(II). The sulfide ions form µ2 or µ3 bridges. The external ligands are always coordinated to the irons. Naturally occurring ligands always include, and predominantly so, S, thiolato side groups from cysteine; and occasionally N, imidazole side groups from histidine, or O, carboxylato side groups from aspartate. Selenium (selenide or selenocysteine) so far has not been found to be a part of naturally occurring iron-sulfur clusters. In some proteins one, and not more than one, of the iron ions can be coordinated by nonprotein ligands, e.g., water. Through strong exchange coupling the Fe/S core ions form a single paramagnetic entity whose ground state has a well-defined system spin S.