This is a survey on the function of parallel visual pathway with a special emphasis on its clinical implications. It is based on data in the literature and own results of our group. The paper primarily deals with the X, Y, W pathways and by the magnocellular, parvocellular and koniocellular visual pathways characterized by cells of various size as well as by nerve fibers of various thickness. Electrophysiological, microelectrode recording of single-unit activity makes the distinction between the pathways available in animal model. Much more difficulties arise if we intend to characterize the pathways in humans or to detect the selective damage of one of these pathways in patients. The non-invasive diagnostic methods that could be used in the diagnosis are detailed here, too. Finally, the neurological, ophthalmological and psychological diseases are discussed in which a selective damage of any visual pathway is suspected. Summing it up, the survey provides evidences for the introduction of the novel concept of parallel pathways into the diagnostic aspects of ophthalmology, neurology and psychiatry.