The ferric uptake regulator (Fur) protein, as originally described in Escherichia coli, is an iron-sensing repressor that controls the expression of genes for siderophore biosynthesis and iron transport. Although Fur is commonly thought of as a metal-dependent repressor, Fur also activates the expression of many genes by either indirect or direct mechanisms. In the best studied model systems, Fur functions as a global regulator of iron homeostasis controlling both the induction of iron uptake functions (under iron limitation) and the expression of iron storage proteins and iron-utilizing enzymes (under iron sufficiency). We now appreciate that there is a tremendous diversity in metal selectivity and biological function within the Fur family which includes sensors of iron (Fur), zinc (Zur), manganese (Mur), and nickel (Nur). Despite numerous studies, the mechanism of metal ion sensing by Fur family proteins is still controversial. Other family members use metal catalyzed oxidation reactions to sense peroxide-stress (PerR) or the availability of heme (Irr).