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Introduction ROBERT SHAPLEY The visual interface between human visual perception and patterns of light in the world is the photoreceptor layer of the retina. All our experience of the appearance of objects and spatial or temporal patterns of illumination is filtered through the receptor array. All visual function thus depends on the characteristics of photoreceptors for example, their responsiveness to different wavelengths of light, the gain and regulation of the biochemical cascade kicked off by the absorption of a photon of light, and the spatial arrangement of photoreceptors within the photoreceptor layer. Our keynote speaker, William Miller, has contributed major insights to two of the areas of photoreceptor function: the biochemical cascade of phototransduction and the nature of photoreceptor assays. Dr. Miller was one of the first scientists to look at cyclic nucleotides as candidates for the internal messenger molecule that mediates transduction. The great discov- eries of recent years about cyclic GMP, phosphodiesterase, transduction or G-protein, and the regulatory roles of calcium and guanylate cyclase are all results of experiments and models inspired by the initiatives of Dr. Miller and colleagues. At the Frontiers of Vision symposium, Dr. Miller spoke briefly about optical effects in green rods and devoted the major part of his address to the topic of transduction. His talk is an introduction to a great problem, now apparently solved, by one of the great masters who contributed to its solution. Dr. Miller's insights into the major issues of transduction are penetrating. His summary of the process is a useful introduction to the rest of the symposium. 1