Light falling on photoreceptors is transformed by retinal circuitry into a pattern of action potentials that ganglion cell axons convey to the visual centers in the rest of the brain. This process begins with phototransduction, a biochemical cascade that ultimately regulates the opening and closing of ion channels in the membrane of the photoreceptor’s outer segment, and thereby the amount of neurotransmitter the photoreceptor releases. Two systems of photoreceptors—rods and cones—allow the visual system to meet the conflicting demands of sensitivity and acuity, respectively. Retinal ganglion cells operate quite differently from the photoreceptor cells. Two distinct classes of ganglion cells convey information about luminance increments and decrements (light and dark). Retinal circuits dynamically regulate the sensitivity of retinal ganglion cells and adjust their operating range to permit changes in activity to convey information over a broad range of stimulus conditions. The center–surround arrangement of ganglion cell receptive fields enhances the activity of those ganglion cells that carry the most information about object boundaries in the visual scene.