In this introductory article, we summarize the evidence from humans and animal models on the shaping of postnatal visual development by focused binocular input. When balanced input is missing during a sensitive period, deficits emerge, including seemingly permanent impairments in visual acuity that are labeled amblyopia. Rodent models have identified neurochemical changes that control the onset of such sensitive periods and molecular and structural brakes that lead to the diminution of the plasticity thereafter. Both animal and human studies of amblyopia have recently identified exciting ways to remediate vision in adulthood that bear some similarity to the interventions that have proved successful in promoting recovery from stroke. © 2012 Wiley Periodicals, Inc. Dev Psychobiol 54:224-238, 2012.