A brief history of the discovery of photosynthetic phosphorylation by chloroplasts and bacterial chromatophores is presented. Arnon early introduced the terminology of 'Cyclic' and 'Non-cyclic photophosphorylation' and 'Cyclic' and 'Non-Cyclic electron transport' to the processes observed in illuminated chloroplasts. He made major contributions to the elucidation of these processes and stressed their great biological significance. Investigations of the electron transport components of chromatophores have led to the isolation, purification and crystallization of bacterial reaction centers. The development of three-dimensional molecular structures, and the characterization of their electron transfer components have provided a great deal of information about the early reactions of bacterial photosynthesis. The electron transfer schemes presented clearly support the 'cyclic' nature of light-induced electron transfer. Recent developments in the understanding of ATP synthesis in oxidative phosphorylation by mitochondria and in photophosphorylation by chloroplasts and bacterial chromatophores are discussed.