Sequential accommodation of single electrons by the unpaired orbitals of dioxygen yields oxygen radicals Symbol, hydrogen peroxide (H2O2), hydroxide radicals (OH·), and finally water (H2O). Fe2+ catalyses the formation of the most reactive hydroxide radical from hydrogen peroxide and thus contributes substantially to the toxicity of oxygen. Insolubility of Fe3+ demands the incorporation of iron into transferrin, lactoferrin, ferritin, iron-sulphur clusters, and heme. Bacteria and fungi synthesize low molecular weight compounds, termed siderophores, which are secreted and used to transport Fe3+ into the microbial cells. Iron is economically used and iron toxicity is minimized by the synthesis of siderophores and ferric siderophore transport systems, and by induction of transport gene transcription by certain Fe3+-loaded siderophores. When cells contain sufficient iron, Fe2+-loaded Fur protein and Fe2+-loaded DtxR protein repress gene transcription in Gram-negative bacteria and in most Gram-positive bacteria, respectively. In a recently discovered novel transcription control mechanism, ferric citrate and ferric pseudobactins induce transcription of the iron transport systems by binding to cell surface receptor proteins without entering the cells. Cytoplasmic sigma factors are activated by a signaling device that involves a protein in the outer membrane and a protein in the cytoplasmic membrane. Both proteins extend into the periplasm to transduce the signal through the space between the two membranes. Intracellular iron homeostasis secured by regulation of iron uptake prevents excessive oxidative stress, which could otherwise overcome the cellular defence and repair systems and kill the cells.