Recent advances in research on biochemical aspects of photosynthesis has brought with it voluminous reviewing activity (4,5,18,37,44,52,75,78,79,112,133,178,218,230,231). It was decided, therefore, to limit the coverage of this review and discuss subjects with which the authors have been most closely associated. Bacterial photophosphorylation will not be dis cussed. GENERAL CHARACTERISTICSPhotophosphorylation in vivo.-Two approaches have been used re cently to determine whether photophosphorylation does indeed contribute significantly to the ATP produced in photosynthetic cells [for a review of the earlier, rather contradictory data, see Arnon (1) and Kandler (126)].In the first direct approach, ATP levels, or the rate of phosphate uptake, were determined under conditions where the only process capable of pro ducing ATP was photophosphorylation. Thus, Forti & Parisi (71) and San tarius & Heber (194) showed that the ATP level in leaves doubled on i1!u mination. In the former the increase was observed even under anaerobic conditions and in the presence of CMU. Similar conclusions were reached by Simonis and his co-workers who measured the rate of phosphate uptake in algae und er a variety of conditions (206).In the second, indirect approach, reactions which depend upon a steady supply of ATP were measured. Maclachlan & Porter (146) showed that