Functioning and efficient cell signalling is vital for the survival of cells. Over the years various components have been identified and recognised as crucial for the transduction of signals in cells. Many of the mechanisms allow for a relatively rapid switching of signals, on or off, with common examples being the G proteins and protein phosphorylation. However, recently it has become apparent that other modifications of amino acids are also important, and this includes reactions with nitric oxide, for example S-nitrosylation, and of particular relevance here, oxidation of cysteine residues. Such oxidation will be dependent on the redox status of the intracellular environment in which that protein resides, and this will in turn be dictated by the presence of pro-oxidants and antioxidants. Here, the chemistry of redox modification of amino acids is introduced, and a general overview of the role of redox in mediating signal transduction is given.