Four decades ago, when Lou Duysens was about to start his work on fluo rescence and energy transfer in photosynthesis that would lead to his thesis [1], very little was known about the molecular mechanisms of photosyn thesis, certainly from our present-day point of view. However, this state of affairs would rapidly change in the ensuing years by the introduction of modern physical and biochemical techniques. Especially the field of optical spectroscopy, on which the work of Duysens had such a significant impact, has proved to be one of the most fruitful techniques in the study of primary processes and electron transfer reactions in photosynthesis. Duysens' thesis established the role of energy transfer in photosynthesis and also showed for the first time the existence in photosynthetic bacteria of light-induced absorbance changes of what is now known as the primary electron donor P-870. Subsequent studies by the same method demonstrated the photo-oxidation of cytochromes, both in bacteria [2] and in algae [3,4] and of the absorbance changes [3] that were later found to be due to electro chromic band shifts of antenna pigments. Measurements of cyto chrome kinetics in light of various wavelengths led to the concept of two photosystems in green plant photosynthesis [5], whereas a study of the factors affecting the fluorescence yield of chlorophyll gave the first infor mation on the electron acceptor Q of photo system II [6].

Progress in photosynthesis research is strongly dependent on instrumentation. It is therefore not surpr- ing that the impressive advances that have been made in recent decades are paralleled by equally impressive advances in sensitivity and sophistication of physical equipment and methods. This trend started already shortly after the war, in work by pioneers like Lou Duysens, the late Stacy French, Britton Chance, Horst Witt, George Feher and others, but it really gained momentum in the seventies and especially the eighties when pulsed lasers, pulsed EPR spectrometers and solid-state electronics acquired a more and more prominent role on the scene of scientific research. This book is different from most others because it focuses on the techniques rather than on the scientific questions involved. Its purpose is three-fold, and this purpose is reflected in each chapter: (i) to give the reader sufficient insight in the basic principles of a method to understand its applications (ii) to give information on the practical aspects of the method and (iii) to discuss some of the results obtained in photosynthesis research in order to provide insight in its potentalities. We hope that in this way the reader will obtain sufficient information for a critical assessment of the relevant literature, and, perhaps more important, will gain inspiration to tackle problems in his own field of research. The book is not intended to give a comprehensive review of photosynthesis, but nevertheless offers various views on the exciting developments that are going on.