The critical period in which human visual acuity may be lost after visual deprivation can be deduced by studying children with amblyopia which follows uncomplicated unilateral cataract and the subsequent aphakic blur. The visual acuity at the first contact lens fitting increases linearly on a log scale as a function of the age at onset between 3 and 10 years. Correction before 4 months of age produces less visual loss but we cannot specify the function within this period. If the eye is deprived between 6 and 30 months of age finger counting is the best visual acuity achieved. Deprivation commencing between the ages of 3 and 10 years differs only in that vision reduces at a slower rate and is more likely to respond to total full-time occlusion. Similarly, some unilateral high myopes responded well and three unilateral aphakes who are blind in the other eye slowly achieved good vision. Part-time occlusion and Cam treatment were seldom effective. Children appear to have a critical period which continues for longer than expected from animal models.