Advances in our knowledge of the developing visual system have come experimentally from animal studies and, more empirically, from clinical observations. Recently morphological techniques used in experimental animals have been applied to the study of human nervous tissue at various ages. It can be observed that the human visual system is very immature at birth, at the level of the retina, the lateral geniculate nucleus (LGN) and the visual cortex. In the LGN and cortex there is an increase in post-synaptic surfaces (dendrites and spines) during the first postnatal months with a subsequent decrease to a lower, "adult" level by the second year. Synaptogenesis in the cortex is rapid after birth, with a maximum synaptic density at about 8 months. Thereafter synapses are eliminated to reach "adult" levels at about 11 years. In the monkey similar phenomena have been observed, but with a time-scale about four times shorter. In both monkey and man these developmental changes manifested by an initial overgrowth of pre- and postsynaptic elements and a subsequent "pruning" to mature levels correspond to a period of physiological and behavioural changes in visual function. This period represents a critical phase in normal development at a time when the risk of the development of amblyopia is at its highest.