 The glutathione system plays a critical role in cellular protection against oxidative stress and other environmental factors. In particular, cyanobacteria have been found to be particularly adept at utilizing the glutathione system to protect themselves from the reactive oxygen species produced during their active photosynthetic metabolism. The glutathione system is composed of three main components, glutathione itself, its derivatives ergothionine and phytokeletons, and the enzymes involved in its production and breakdown. Cyanobacteria produce these compounds, allowing them to use the glutathione system to protect themselves from oxidative damage. Additionally, cyanobacteria can synthesize the thiol-less glutathione homologs of thalamate and norothalamate, which can be used as biomarkers of certain diseases in humans. This article was authored by Corinne Kassier-Chauvat, Fannie Marceau, Sandrine Farsi, and others.