Electrophysiologic testing of the visual system requires primarily the ERG and the VEP. The flash electroretinogram provides information about the outer retina only. The pattern electroretinogram is derived from both the outer retina and the innermost retinal layers including the ganglion cell layer. The VEP is based on electrical information recorded from the visual cortex in response to stimulation of the retina. Thus, the integrity of the entire visual pathway can be tested. Localizing ability of the VEP is limited. Since the visual cortex is heavily weighted by representation of the central retina, peripheral lesions, including those producing peripheral visual field abnormalities that do not impinge upon central fixation, may produce relatively little disturbance of the VEP. As with most tests, electrophysiologic studies of the visual system must be placed in context of the entire examination, including the patient's history and neurologic and especially neuro-ophthalmologic evaluation. Electrophysiologic testing has three main uses in neurology. Pattern-reversal VEPs may be useful in detecting hidden visual loss in multiple sclerosis; VEPs and ERGs can distinguish function from organic visual loss; and VEPs and ERGs can be useful in the diagnosis of visual loss in nonverbal patients, especially in children.