The effects of visual deprivation in 15 patients (14 children and one adult) were studied. The age of onset of deprivation and the significance of final visual outcome are discussed. Results indicate that a critically sensitive period in visual development occurs between approximately four months and three years of age. A period of plasticity, when the effects of deprivation are more responsive to therapy, follows. Age of onset of deprivation was found to be most important, but in addition loss of accommodation in association with deprivation may be a significant factor in the development of amblyopia.