Upper airway anatomy influences the pathophysiology of obstructive sleep apnea (OSA). Understanding this anatomy helps dissect the pathophysiologic factors involved in OSA. During sleep, the complex interaction among pharyngeal dilator tone, upper airway narrowing, changes in lung volume, arousal threshold, and respiratory control instability plays an important role in OSA and leads to considerable heterogeneity in the mechanism of OSA. This complexity also helps to explain the known risk factors for OSA, such as obesity, sex, and age.