This book collects all the lectures presented during the NATO Advanced Study Institute on Photoreception and Sensory Transduction in Aneura1 Organisms, held in Villa Le Pianore (Versi1ia, Italy), September 3-14, 1979. In order to publish the lectures in the shortest possible time, we had to make the decision not to include the free communications, the informal seminars, and the panel discussions, notwithstanding their very high scientific level and interest. Only the final panel discussion has been summarized by Prof. W. Haupt (whose effort we gratefully acknowledge), because it gives a comprehensive view of the state of the art in this field. The ASI was intended to be a high-level course, characterized by an interdisciplinary approach to the problem of photoreception and photosensory physiology in aneural organisms, bringing together scientists from different fields and specializations. We hope that these characteristics are reflected in the content of the book, which is meant to be both an advanced textbook for researchers and students entering the field and a critical overview of the problems of photosensory transduction in aneura1 organisms. The topics pre sented range from a phenomenological description of the different photomotile responses in various microorganisms to a discussion of the molecular processes involved in the primary events of photo reception as well as in the subsequent steps of the transduction chain.