The study of dreams is a study of a group of epiphenomena associated with states of diminished awareness. No matter whether such dreaming occurs in natural sleep, in states of reverie interspersing normal levels of consciousness or in pathologically or pharmacologically induced aberrations of consciousness, it would appear that the principle that dreams do not occur other than at levels of lessened awareness is generally accepted. From the point of view of neurophysiology, it also seems reasonably clear that the formation of the visual and auditory hallucinations, illusions and delusions forming the manifest formal content of dreams is dependent upon fairly precise cerebral localization, the temporal lobe and Brodman's area 19 being especially important in these respects (Penfield and Erickson, 1941). Psycho-physiologically a number of attempts have been made to assess the nature and quality of stimuli normally inciting dreams. From Maury (1861) onward the effects of various physical stimuli (Klein, 1930), of auto-suggestion (Arnold-Forster, 1921), of post-hypnotic suggestion (Prince, 1939) and of projection phantasies (Malamud and Linder, 1931) have been noted. Psychologically the dynamics underlying the mechanisms of the dream instinct have been investigated especially by Freud and his followers. Freud's opinion that the dream represents an attempted wish fulfilment has been modified by Jung, who emphasizes its compensatory function, by Adler, who conceived of the dream as a method for testing possible future situations in phantasy, and by Stekel (1943), for whom the dream was a symbolic manifestation of the “internal conflicts” of the individual. Finally, the observations of Monge (1929) concerning the increased incidence of disturbing dreams when dwellers at high altitudes suffer from chronic mountain sickness, and of McFarland (1937) on the changing dream habits of a party of mountaineers in the Chilean Andes as they ascended steadily to ever-increasing altitudes (and hence to progressively greater degrees of anoxic anoxia), are of immediate interest. They are in direct line with the findings of anoxaemia in sleep (Lovett Doust and Schneider, 1951), and suggest a positive relationship between the manifest content of dream patterns, emotional tension, reduced awareness and anoxaemia.