Ferredoxin (Fd)-dependent cyclic electron transport through photosystem I (PSI) was first discovered to be cyclic photophosphorylation coupled with ATP synthesis in the chloroplast. Although pioneer studies provided important information, the physiological significance of this electron transport has been underestimated in C3 plants. The discovery that the Arabidopsis pgr5 (proton gradient regulation) mutant shows impaired Fd-dependent cyclic electron transport was the first piece of evidence of molecular-scale information on this electron transport pathway and its physiological function. The recent advance of techniques for measuring the activity of cyclic electron transport in vivo has allowed a re-evaluation of its activity using wild type and pgr5. In this review, we discuss the major role played by Fd-dependent cyclic electron transport, especially in 1) induction of thermal dissipation, 2) contribution to ATP synthesis and 3) photoprotection of PSI.