In response to high light, photosynthetic organisms change their photosynthetic apparatus to maintain a balance between energy supply (light harvesting and electron transport) and consumption (CO₂ fixation), and to avoid the photodamage. This widely-observed and important phenomenon, termed acclimation to high light, has been extensively investigated from the physiological point of view. However, its molecular mechanism remains still unknown. Today, the molecular approach to light acclimation in cyanobacteria is showing rapid progress taking advantage of the simplicity of gene manipulation, the simple cellular structure and metabolic processes, and sequence information available for the entire genome of a cyanobacterium Synechocystis sp. PCC 6803. In this article, the latest information about the molecular mechanism of (1) adjustment of antenna size, (2) regulation of photosystem stoichiometry, (3) acclimation of Photosystem II reaction center and (4) scavenging system for reactive oxygen species in cyanobacteria upon a shift to high light are overviewed. Further, (5) relationship between high light and low temperature and (6) the putative genes which may be involved in high-light acclimation are discussed.