The light reactions in photosynthesis convert light energy into chemical energy in the form of ATP and drive the production of NADPH from NADP+. The reactions involve two types of electron flow in the chloroplast. While linear electron transport generates both ATP and NADPH, photosystem I cyclic electron transport is exclusively involved in ATP synthesis. The physiological significance of photosystem I cyclic electron transport has been underestimated, and our knowledge of the machineries involved remains very limited. However, recent genetic approaches using Arabidopsis thaliana have clarified the essential functions of this electron flow in both photoprotection and photosynthesis. Based on several lines of evidence presented here, it is necessary to reconsider the fundamental mechanisms of chloroplast energetics.