Preoxygenation is a technique used as a safety measure to prevent oxygen desaturation during various airway management procedures. It essentially does so by increasing the oxygen reserves in the body, which help maintain the oxygen saturation during periods of apnea. Although preoxygenation is being practiced for many years, its role in providing safety against desaturation, especially during management of difficult airway, is increasingly being emphasized over the last two decades. Preoxygenation is also recommended at emergence from anesthesia. The residual effect of anesthetic agents and opioids may cause hypoventilation. This chapter covers the physiological basis of preoxygenation, oxygenation during airway management, markers of preoxygenation, techniques of preoxygenation, preoxygenation in special situations, and potential risks of preoxygenation.