Over the past decade and a half, there have been great advances in our understanding of how light is transduced into electrical signals by the retinal rod and cone photoreceptors in vertebrates. One essential feature of these sensory neurons is their ability to adapt to background illumination, which allows them to function over a broad range of light intensities. This adaptation appears to arise mostly from negative feedback on phototransduction that is mediated by calcium ions. Recent work has suggested that this feedback is fairly complex, and involves several pathways directed at different components of phototransduction. From direct measurements of these feedback pathways in rods, it is possible to evaluate their relative contributions to the overall sensitivity of the cell. At the same time, these feedback mechanisms, as currently known, appear to be sufficient for explaining the change in sensitivity of rods during adaptation to light.