Redox-sensitive cysteine residues sense and transduce changes in cellular redox status caused by the generation of reactive oxygen species and the presence of oxidised thiols. Oxidation of such cysteines is converted into signals that control cell regulatory pathways and induction of gene expression. A variety of proteins, including transcription factors, molecular chaperones and protein tyrosine phosphatases, are regulated via redox processes. Common mechanisms underlie the sensitivity of cysteines to redox, such as proximity to polar and charged groups, and signal transduction is exerted via conformational changes that are conferred by the formation of disulfide and cyclic sulfenamide covalent bonds, and sulfenic and sulfonic acids.