Recent studies on the redox behaviour of cysteine residues in peptides and proteins have dramatically changed our perspective of the amino acid's role in biocatalysis, intracellular redox sensing and cell signalling. Cysteine sulfinic acid formation in proteins, for example, has long been viewed as an irreversible 'overoxidation' process that might lead to loss of activity, especially under conditions of oxidative stress. Within the last year, several research groups have independently shown that sulfinic acids can be reduced to thiols in vivo. An enzyme with sulfinic acid reductase activity, called sulfiredoxin, has been isolated from yeast and a gene encoding a human analogue has been identified in the human genome. Reversibility of sulfinic acid formation opens the door to a range of yet unexplored redox cycles, cell signalling processes and reduction mechanisms. These cysteine-based redox processes will be of enormous interest to chemists, biochemists, biologists and the medical community alike.