Irradiance changes dynamically with time in the field (fluctuating light; FL). It is also noted that sun light contains far-red light (FR, 700 - 800 nm) as well as photosynthetically active radiation (400 - 700 nm). Since FR itself cannot drive photosynthetic O₂ evolution, importance of FR in photosynthesis tends to be neglected. However, by driving photosystem I (PSI), FR alleviates the acceptor-side limitation and imposes the donor-side limitation on the electron flow in PSI, thereby contributing to maintenance of oxidized P700 (P700⁺). We have been focusing on the roles of FR in photosynthesis. We have reported that FR effectively protects PSI against the FL-induced damage via the enhancement of the non-photochemical energy dissipation by P700⁺ and the PSI cyclic electron flows in the Arabidopsis, rice and tobacco. In more moderate FL, the CO₂ assimilation rate upon a transition from high- to low-light was significantly enhanced by FR, resulting from the increased photosystem II (PSII) photochemistry and the rapid relaxation of the non-photochemical quenching in this phase. In this presentation, we introduce how FR regulates photosynthesis.