If amblyopia is defined as reduced visual acuity in an eye which appears normal on examination, then it is necessary to distinguish between several related, similar, or different conditions that fit this definition. Six years ago I proposed a classification of amblyopia in which the various factors leading to its development were enumerated in the light of our clinical knowledge at that time (von Noorden, I967). Meanwhile, with the advent of modern neurophysiological techniques, amblyopia has become a favourite research subject of neurophysiologists and neuroanatomists, and the effect of abnormal visual experience early in life on visual functions of the adult organism is currently being explored on a large scale in lower animals and in monkeys. Some of this information has direct bearing on our concept of amblyopia as it occurs in human patients. The purpose of this paper is to discuss the extent to which our conventional concept of the various factors that cause amblyopia in humans may have to be modified in light of this new information. This discussion will be limited to amblyopia in association with strabismus, anisometropia, and form vision deprivation; organic amblyopia and that associated with nystagmus, achromatopsia, or nutritional deficiencies will not be considered. Let us briefly review the factors that have been held responsible for the production of each type of amblyopia.