Electroretinography is an important objective procedure that is used to assess the outer retina and follow the progression of and recovery from retinal disorders. This procedure is more sensitive than other diagnostic techniques, such as ophthalmoscopy, for determining subtle or early alterations in the outer retina. Electroretinography cannot, however, assess vision because an electroretinograpn (ERG) may be normal in dogs and cats with cortical blindness or early stages of glaucoma. If retinal dysfunction is known or suspected, an ERG may be necessary. This two-part presentation provides general practitioners with information about this relatively noninvasive electrodiagnostic procedure in order to assist them in assessing the need for referral to a veterinary ophthalmologist or neurologist. Part I reviews the morphologic and physiologic characteristics of the retina; Part II will examine electroretinographic technique, interpretations, and indications.