Photosynthesis--the capture of light energy by living organisms -is a simple enough concept, but its investigation draws on the resources of disciplines from all fields of science. The aim of this text is to provide a clear, stimulating and essentially affordable coverage for undergraduate students of biology. The activity of science is debate and practical experiment; its product is a body of propositions which at any given time reflects the judgment and prejudices of those taking part. The value of a proposition is related to the conceivable alternatives, and writing it down without its context creates the false impression that science progresses by compilation of an increasing list of absolute truths. It does not; the facts and figures pres­ ented in the following pages have no intrinsic value unless they can be used by the reader to support an argument or point of view. In short, the reader is urged to respond 'So what?' to every item. Secondly, ideas-like other foods-should be date-stamped; science is inseparable from its history. I have set out time-charts to represent the evolution of our understanding in certain areas. I have assumed that the reader is pursuing a course with a content of biochemistry, microbiology and plant science, or has access to basic texts. I have assumed also that common methods such as spectrophotometry, chromatography and electrophoresis, as well as the techniques of mol­ ecular biology, will be either part of the same course or in active use nearby.