© Springer Science+Business Media, LLC, part of Springer Nature 2019. It is the functioning of efficient cell signaling which is vital for the survival of cells, whether it is a simple prokaryote or a complex eukaryote, including both animals and plants. Over many years various components have been identified and recognized as crucial for the transduction of signals in cells, including small organic molecules and ions. 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, it has become apparent that other amino acid modifications are also vitally important. This includes reactions with nitric oxide, for example S-nitrosation (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, either produced by the cell itself or from the cell’s environment. 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.