There is very little in this eleventh volume of Topics in Photosynthesis which could have been written when the first volume was published fifteen years ago. Advances over the last decade have been spectacular, most particularly in our understanding of the photosystems that is the subject of this volume. After a comparative introducution of bacterial and plant photosystems, the book begins with a consideration of what is theoretically possible in energy conversion. This is followed by light harvesting in photosystems II, followed by its molecular biology, protein engineering, thermoluminescence, photoinhibition, the effect of herbicides and heat shock, and, most important function of all and one about which so little is yet understood at the molecular level, the process by which it evolves oxygen. The last three chapters deal with the equivalent processes of photosystem I. The whole volume tells the story of a natural system of incredible ingenuity and complexity, but which as the chapters unfold, is seen to be within our grasp and eventual ability to comprehend.