CP12 is a small, redox-sensitive protein, representatives of which are found in most photosynthetic organisms, including cyanobacteria, diatoms, red and green algae, and higher plants. The only function that has been clearly demonstrated for CP12 in any organism is in the regulation of the C3 photosynthetic cycle by mediating the formation of a complex between glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and phosphoribulokinase (PRK) in response to changes in light intensity. Under low light the GAPDH-PRK-CP12 complex forms and the activity of both enzymes is reduced; under high light conditions the complex dissociates, mediated by thioredoxin, resulting in increased GAPDH and PRK activity. Although the role of CP12 in the non-enzymatic, redox-mediated formation of a multiprotein complex is clear, a number of studies now provide evidence that the CP12 proteins may have a wider role. CP12 is expressed in non-photosynthetic A. thaliana tissues, and anti-sense suppression of tobacco CP12 disrupts a variety of developmental processes. Furthermore, in addition to the higher plant genomes which encode up to three forms of CP12, analysis of cyanobacterial genomes has revealed that not only are there multiple forms of the CP12 gene, but that in these organisms CP12 is also found fused to cystathionine-β-synthase domain containing proteins. In this review we present the latest information on the CP12 protein family and explore the possibility that CP12 proteins form part of a redox-mediated metabolic switch, allowing organisms to respond to rapid changes in the external environment.