This volume or1g1nates from a NATO Advanced Study Institute on "Photoreceptors" dedicated to M.G.F. Fuortes, held in Erice 1-12 July 1981. The lectures given at the course provided a general review of the photoreceptors functions in both vertebrate and inver- tebrate eyes. Elaborating on the most recent hypotheses the lec- tures also added new and interesting details. In order to preserve the novelty and freshness of the subject matter and thus ensure the usefulness of the volume, the authors in their written contributions emphasize more specific findings of their current research rather than the tutorial nature of the lectures actually presented. The contributors of this volume wish to dedicate their papers to the memory of Mike Fuortes, who has been for many of them an inspiring collegue and friend. Five papers that do not strictly pertain to the topics of the course are also included in the volume, the authors could not attend the meeting, but wanted to contribute an article to this memorial volume.