One of a continuing series on Frontiers of Visual Science, this short volume contains papers presented at a recent symposium. These papers describe techniques for assessing the structure and function of photoreception, both in isolation and in aggregation as the "receptor mosaic" of the retina. Also described are dynamic and spatial sampling properties of photoreceptors, with an emphasis on techniques of measurement and associated models of retinal function. This volume should be of interest to basic vision scientists, clinical ophthalmologists, workers in human factors, and computer scientists curious about the visual apparatus of biological systems.