The passive human upper airway (UA) is a collapsible tube with a relatively high compliance. At atmospheric luminal pressure, its cross-sectional area varies considerably. Subjects in whom the pharynx is closed, or nearly closed, at near atmospheric pressure require an upper airway dilating force to maintain adequate ventilation. During wakefulness pharyngeal dilator muscles (dilators) provide the necessary force to permit an adequate flow, regardless of how collapsible the pharynx is. This dilator activity is substantially lost at sleep onset. Subjects in whom the passive UA cannot permit adequate ventilation must recruit dilators through reflex mechanisms if they are to remain asleep. Recent studies have shown that activation of the muscles that open the airway in the course of obstructive apneas persists to a variable degree after the relief of obstruction, evidencing the presence of memory for prior activation in the the brain centers that supply the dilator muscles.