A MBLYOPIA is a catchall term used for unexplained reduction of visual acuity. "Unexplained" means that, in spite of any refractive error being neutralized with lenses and in the absence of detectable eye disea,se, the acuity of the eye is still below normal. The acuity criterion for amblyopia is ill defined, but a standard ofte,n adopted is 20/40 or worse. Amblyopia almost always affects only one eye. Reduced monocular visual acuity is but one feature of amblyopia. The amblyopic person usually has difficulty in aiming and moving the eye as well as in seeing an object when it is surrounded by other forms. These difficulties cause him to make peculiar responses on a monocular acuity test-responses which aid the experienced examiner in detecting amblyopia but may lead the inexperienced examiner to pass, the amblyopic person as having normal acuity. On an acuity test, the person with amblyopia signals himself by making errors over an abnormally large range of letter sizes (even though he may correctly read some of the smallest letters) . He tends to miss letters in the middle of the row and more often correctly reads those at the ends. He frequently reads letters out of order, and he has more difficulty reading letters when their spacing is less than one letter apart. Most persons with amblyopia have a significant refractive error, but neutralization of the error with lenses permits them to have e,ssentially normal acuity when both eyes are open. Their everyday perception of depth is generally unaffected because nearly all cues to depth perception are monocular, and binocular cues (which many amblyopic persons lack) add little in ordinary visual situations. Since amblyopia, affects centra,l and not peripheral vision, the extent of the binocular field of vision is not limited by amblyopia. If the amblyopia is associated with an "eye turn" (strabismus), as is often the case, then the lateral extent of the binocular visual field tends to be smaller for "crossed eye" (convergent strabismus) and larger for "wall eye" (divergent strabismus). These characteristics of amblyopia point up the difficulties amblyopic persons may have when attempting to see with the affected eyea temporary situation if the good eye is simply covered, a more permanent problem if the norma,l eye is lost or severely damaged. The visual difficulties of the amblyopic person are thus more potential than actual. The most universally accepted view of the origin of amblyopia is an amalgamation of ideas proposed by Claude Worth (1) and Bernard Chavasse (2). In substance, these authors stated that amblyopia is acquired in childhood as a consequence of not using one eye because it is "turned" (strabismus) or because it has a refractive error very different from that of the other eye (anisometropia). Acceptance Dr. Flom is associate professor of physiological1 optics and optometry, and Dr. Neumaier is a clinic instructor, University of California School of Optometry, Berkeley. This paper is an elaboration of one presented to the American Academy of Optometry on December 12, 1964, in Columbus, Ohio. The stuldy was supported in part by a Public Health Service grant (NB 4242) from the National Institute of Neurological Diseases and Blindness. Data were made available to the study through efforts of James A. Collins, assistant superintendent of schools, Lafayette, and Kay Dorris, Lafayette health consultant (Lafayette School District data); Dr. Henry B. Peters, director of Berkeley's optometry school clinics (Orinda Union School District data); and Dr. Kenton E. Kerr, clinic instructor at the school of optometry (optometry clinic data).