The electroretinogram (ERG) is used to measure electrical activity of various cell types in several layers of the retina, including rods, cones, inner retinal cells, and ganglion, in response to a light stimulus. ERG therefore provides objective information about visual system function and can provide diagnostic information for inherited and acquired retinal disorders. It can also provide objective measurements of disease progression, including development of retinal toxicity due to medications, and treatment response.