This interdisciplinary clinical project will determine whether monitoring diagnosis and therapy with quantitative visual acuity measurements has an effect on clinical management and outcome of treatment in preverbal patients at risk of amblyopia. In effect the studies will attempt to bring a part of the standard clinical examination of older children to the examination and treatment of preverbal children. To be included in the project are children with the most common causes of amblyopia: strabismus, significant anisometropia, and media opacities. Amblyopia represents the most prevalent cause of visual pathology in children, affecting 5% of Americans. Changes in the visual system associated with amblyopia tend to become irreversible if treatment is not begun early in the putative critical period. Treatment, typically including patching of the dominant eye, is therefore most effective if initiated in early childhood, often during the preverbal years when standard acuity testing is not possible. Both diagnosis and monitoring of treatment in amblyopia require precise monocular acuity measures. Without such monitoring, patching can be harmful and can actually cause amblyopia in the eye being patched. Preferential looking (PL) is one effective quantitative acuity measure used in preverbal children at risk of amblyopia. It will be used in three concurrent studies. Study A involves two patient groups: an Already Tested Group, managed without PL, will be compared to a prospective Preferential Looking Group. Studies B and C will each include 2 prospective patient groups: a Known Acuity Group will be managed with PL acuities as a guide while a Standard Treatment Group will be assessed using non-quantitative clinical testing. Since not all preverbal children can be tested with present versions of PL, it is recognized that there is a need to also offer treatment to these patients. To make the test available to the widest possible preverbal patient population, attempts to improve the technique are contemplated in Study C. It will compare the testing failures of Study B that are successes with the modified PL test. Normal perverbal children will also be studied. Assessment of visual outcome will be made in each group at the age standard acuity measurement becomes feasible. The impact PL testing has on the treatment of each patient group will also be evaluated.