Bacteria adapt to the presence of reactive oxygen species (ROS) by increasing the expression of detoxification enzymes and protein and DNA repair functions. These responses are co-ordinated by transcription factors that regulate target genes in response to ROS. We compare three classes of peroxide-sensing regulators: OxyR, PerR and OhrR. In all three cases, peroxides effect changes in the redox status of cysteine residues, but the molecular details are distinct. OxyR is converted into a transcriptional activator by the formation of a disulphide bond between two reactive cysteine residues. PerR is a metalloprotein that functions as a peroxide- sensitive repressor. Oxidation is modulated by metal ion composition and may also involve disulphide bond formation. OhrR represses an organic peroxide resistance protein and mediates derepression in response to organic peroxides. Peroxide sensing in this system requires a single conserved cysteine, which is oxidized to form a cysteine-sulphenic acid derivative.