This volume contains the lectures given at the NATO Advanced Study Institute on "Biophysics of Photoreceptors and Photomovements in Microorganisms" held in Tir- renia (Pisa), Italy, in September 1990. The Institute was sponsored and mainly funded by the Scientific Affairs Division of NATO; the Physical Science Committee and the Institute of Biophysics of National Research Council of Italy also supported the School and substantially contributed to its success. It is our pleasant duty to thank these institu- tions. Scientists from very different backgrounds contributed to the understanding of this fast developing field of research, which has seen considerable progress during the last years. The areas of expertise ranged from behavioral sciences, supported by sophi- sticated techniques such as image analysis or laser light scattering, to spectroscopy, ap- plied, in different time domains, to the study of the primary photoreactions, to electro- physiology, biochemistry or molecular biology, with the aim of analyzing the various steps of the transduction chains and how they control the motor apparatus of the cells. The organisms studied covered a wide range, from bacteria to algae, fungi and other eukaryotes. Thus, the ASI represented a successful opportunity for carrying on and imple- menting an interdisciplinary approach to the study of the biophysical basis of photore- ception and photosensory transduction in aneural organisms, with special attention to the basic phenomena and the underlying molecular events. We hope that this book has caught the spirit in which the ASI was conceived.