While the upper airway normally remains patent during quiet breathing in wakefulness and sleep, patients with obstructive sleep apnea have repetitive periods of upper airway closure during sleep. The upper airway closures usually occur at various sites in the pharynx. The patency of the potentially collapsible pharynx during inspiration depends on the balance between subatmospheric pressure in the pharyngeal airway and airway dilating forces generated by pharyngeal muscles. The pressure required to collapse the upper airway in the absence of upper airway muscle activity, ie, closing pressure, is normally subatmospheric. In obstructive sleep apnea, positive pressures are required to maintain patency of the passive upper airway. The pathophysiologic mechanisms underlying upper airway closures during sleep form the basis for the treatment of obstructive sleep apnea. In general, these treatment modalities attempt to (1) raise the pharyngeal pressure above the closing pressure, (2) decrease the closing pressure, or (3) increase upper airway muscle activity.