Most organisms require several transition metal cations for their metabolism and about 30% of all proteins of a bacterium are metalloproteins. Nevertheless, surplus metal cations are toxic and cells have to maintain a delicate balance of uptake and efflux of these cations. The major aim of this work was the investigation of the homeostasis of the biometals iron, zinc and copper in the Gram-negative γ-proteobacterium Escherichia coli and to elucidate the underlying physiological and biochemical mechanisms. As the first member of the ZIP family of metal transporters that is well known in eukaryotes, the bacterial ZupT-protein was studied in detail. It was demonstrated that ZupT transports not only Zn(II) but also Fe(II), Co(II) and other divalent metal cations into the cytoplasm. Moreover, in addition to a primary Zn(II)-efflux ATPase, a secondary cation diffusion facilitator protein (CDF), ZitB, was shown to be responsible for efficient Zn(II) detoxification. For ZitB essential amino acid residues were identified and also transport kinetics determined. The other CDF-transporter of E. coli, FieF, was studied in detail as well. Interestingly, FieF protects the cell from iron-overload through active efflux of ferrous iron across the cytoplasmic membrane. Thus, FieF is the first bona fide iron-efflux transporter identified in any organism. In addition to two known copper-efflux transporters, a third important copper-detoxification system was identified. CueO is the first bacterial multi copper oxidase that has been analyzed physiologically, biochemically and structurally. In vivo CueO probably not only oxidizes Cu(I) but also the catecholate siderophore enterobactin. In doing so enterobactin-dependent reduction of Cu(II) and thus, generation of toxic Cu(I)-ions within the periplasm is prevented. Therefore, CueO is a connecting link between bacterial copper- and iron-homeostasis.