There were two stages in the history of the studies on ascending reticular activating system of the brain(ARAS). The first stage began with the ARAS discovery by Magoun and Moruzzi and the following investigations using the methods of stimulation and lesion at that time mainly in acute cats. These studies led to the hypothesis of a “diffuse”and “unspecific” ARAS of the brain stem. The second stage was associated with using more precise neurophysiological and histochemical methods mainly in chronically operated free-moving cats and rats. By 2010, the idea of the ARAS as an organized hierarchy of the cerebral “waking centers” distributed along the entire cerebral axis and releasing all the known neuromediators of low molecular weight together with the most important neuropeptides was formulated. To date, the aforementioned hypothesis has been revised again. The glutamatergic activating system has been discovered and described in detail. Presumably, this system is responsible for the appearance of electroencephalogram(EEG) arousal reaction and maintenance of the neocortex in the state of tonic depolarization during wakefulness and rapid eye movement(REM)sleep. Its destruction results in a deep comatose-like state. At the same time, the activity of all other “waking centers” is probably the result of the cortical activation. There were two stages in the history of the studies on ascending reticular activating system of the brain (ARAS). The first stage began with the ARAS discovery by Magoun and Moruzzi and the following investigations using the methods of stimulation and lesion at that time primarily in acute cats. These studies led to the hypothesis of a “diffuse” and “unspecific” ARAS of the brain stem. The second stage was associated with using more precise neurophysiological and histochemical methods mainly in chronically operated free-moving cats and rats. By 2010, the idea of ​​the ARAS as an organized hierarchy of the cerebral “waking centers ” distributed along the entire cerebral axis and release all the known neuromediators of low molecular weight together with the most important neuropeptides was formulated. To date The UG has been discovered and described in detail. Presumably, this system is responsible for th e appearance of electroencephalogram (EEG) arousal reaction and maintenance of the neocortex in the state of tonic depolarization during wakefulness and rapid eye movement (REM) sleep. Its destruction results in a deep comatose-like state. At the same time, the activity of all other "waking centers is probably the result of the cortical activation.