Adenosine triphosphate (ATP), the general fuel of the cell, is the primary product of photosynthesis in most bacteria, and it is one major product, accounting for about 20% of the captured light energy, in oxygenic photosynthesis by cyanobacteria and plants. The proton driven ATP synthase is a paradigmatic enzyme that has it all, an electrical rotary motor being coupled to a rotary chemical generator by a mechanical power transmission. The simplicity and robustness of its construction is just splendid. This chapter describes the interplay of proton pumps and the ATP synthase in photosynthetic organisms and the structure and function of this remarkable enzyme in bacteria and eukarya.