The previous chapter on emerging imaging technologies emphasized new or improved devices that, for the most part, provide anatomic information about the living eye. In this chapter, we will review electrophysiological measures of ocular function. Functional assessment can often confirm and strengthen the anatomical findings. For example, in establishing early hydroxychloroquine (Plaquenil®) toxicity in humans, spectral domain optical coherence tomography (sdOCT) and multifocal electroretinography (mfERG) are frequently used together. Sometimes imaging can provide the most sensitive toxicologic test, such as may be the case for vitreous fluorophotometry and fluorescein angiography in detecting subtle breakdown of the blood-retinal barrier. At other times, functional measures may be the most sensitive. An example from clinical medicine is retinitis pigmentosa, which, early on, may produce only vague subjective symptoms in patients with normal appearing fundi but show a markedly reduced full-field ERG (ffERG). Since the animals used in preclinical toxicologic testing cannot communicate their visual symptoms, a combination of anatomic and functional measures is needed to definitively rule out injury to the visual system.