This chapter discusses the various redox, catalytic, and regulatory properties of Fe-S clusters, as well as their biogenesis. Fe-S clusters contain iron ions both in the ferrous and ferric states and sulfur atoms as sulfide ions. The three systems, nif, isc, and suf, use homologous enzymes for the production of sulfide ions of the Fe-S cluster. One of the main functions of Fe-S clusters in proteins is electron transport. A large number of proteins use Fe in Fe-S clusters as a Lewis acid to perform nonredox catalysis. Among them, one of the most extensively studied is mitochondrial aconitase, which uses a [4Fe-4S] cluster to catalyze the conversion of citrate into isocitrate. Fe-S clusters are sensitive to oxidative damage and when possible, organisms have replaced their Fe-S-cluster-containing enzymes by oxygen-resistant counterparts. Fumarate Nitrate Reduction Regulator (FNR) is used by facultative anaerobic microorganisms as a molecular switch between aerobic and anaerobic metabolisms.