Peroxiredoxins (Prx) constitute a group of recently identified peroxidases that detoxify a broad range of peroxides in distinct subcellular compartments, including chloroplasts. They are ubiquitously expressed in all organisms, i.e. bacteria, fungi, and animals, as well as in cyanobacteria and plants, in which they frequently represent a considerable fraction of total cellular and organellar protein. At least seven prxgenes are expressed in leaves of Arabidopsis. The gene products of four of them are targeted to chloroplasts. Five genes encoding (putative) Prx are found in Synechocystissp. PCC 6803. Based on such circumstantial evidence, as well as biochemical analysis and observations on photosynthetic organisms with modified levels of Prx, it has been established that a subset of Prx plays a role in the context of photosynthesis. The conclusion is further strengthened by studies that showed a modulation of prxgene expression in response to photosynthetic activity. This chapter describes the properties of peroxiredoxins in general and focuses on the role of Prx in protecting the photosynthetic apparatus from oxidative damage and, possibly, in redox signaling in photooxygenic cells.