As far back as the second century, Ptolemy reported the apparent immobility of wheel radius at a certain speed. The psychophysical laws of this flicker fusion phenomenon related to the frequency of the light stimulus were established in 1834-1835 by the Englishman Talbot and by the Belgian Plateau, whose thesis in Liège is described as a landmark in the field. CFF is more a measurement of cortical arousal than of visual functions. In psychophysiology, CFF underwent periods of success and oblivion, at the mercy of researcher's enthusiasm or disappointment. At the turn of this century, Pierre Janet measured CFF in the laboratory of physiology of the Salpêtrière Hospital and demonstrated its decrease 'in hysteria, in states of depression, of lowered tension'. All reviewers of CFF literature have overlooked these observations, reported by Henri Piéron in the 'Melanges dedicated to Monsieur Pierre Janet'. When CFF falls into disgrace, it is because of the variability of its results, due to differences in apparatus and designs of the trials as well as the great number and the intrication of the variables which modify CFF thresholds, among them the nonsensory variables. When CFF is reappraised, as it has been the case in psychopharmacology in recent years, the reason is that it represents a brief, easy and economical measure of vigilance which, under certain conditions, seems to be also reliable, valid and sensitive. In the present monograph, the first in French on CFF, the authors try to analyze the most important contributions of the literature from the standpoint of the most relevant variables: characteristics of the stimulus (light intensity, wave form, wavelength, light-dark-ratio, diameter of the flickering point), test procedure (light vs. dark adaptation, visual angle, continuous vs. discontinuous presentation, monocular vs. binocular vision), influence of various physiological or psychological conditions (pupillary diameter, age, training, IQ; anxiety, depression, motivation etc.). The authors summarize the prerequisite for CFF to measure vigilance or aging in psychopharmacological research. The present Monograph is dedicated to the authors' 'Maîtres', who recently became Emeritus Professors, namely the ophthalmologist Roger Weekers, the pioneer of the clinical application of CFF in Belgium, and the psychiatrist Jean Bobon, who pioneered clinical psychopharmacology in Belgium.