Light controls the formation of plastid ultrastructure and the synthesis of chlorophyll, plastid membrane constituents and Calvin cycle enzymes. A respective light-mediated regulation of the genetic apparatus in the nucleus and the plastid compartment has been reported. Three photoreactions are involved in the regulation: (1) the protochlorophyll (ide) leads to chlorophyll (ide) a photoconversion, (2) the formation of physiologically active phytochrome and (3) light absorption by a blue light receptor (cryptochrome). The chloroplast formation in higer plants is chiefly controlled by active phytochrome, while in lower plants cryptochrome is the prevailing regulatory factor.