Abstract  The Manganese Stabilizing Protein (MSP) of Photosystem II (PSII) is a so-called extrinsic subunit, which reversibly associates with the other membrane-bound PSII subunits. The MSP is essential for maximum rates of O₂ production under physiological conditions as stabilizes the catalytic [Mn₄Ca] cluster, which is the site of water oxidation. The function of the MSP subunit in the PSII complex has been extensively studied in higher plants, and the structure of non-PSII associated MSP has been studied by low-resolution biophysical techniques. Recently, crystal structures of PSII from the thermophilic cyanobacterium Thermosynechococcus elongatus have resolved the MSP subunit in its PSII-associated state. However, neither any crystal structure is available yet for MSP from mesophilic organisms, higher plants or algae nor has the non-PSII associated form of MSP been crystallized. This article reviews the current understanding of the structure, dynamics, and function of MSP, with a particular focus on properties of the MSP from T. elongatus that may be attributable to the thermophilic ecology of this organism rather than being general features of MSP.