Background Preoxygenation is one of the necessary procedures before tracheal intubation, but hadn’t been emphasized and popularized in ICU because of lacking of clinical evidences. Much work of preoxygenation before elective general anesthesia, has been done, but the study in ICU has just begun. Objective The summary discussed the relevant techniques , practice, problems and future development of .preoxygenation in ICU. Content Preoxygenation in critically ill patients requiring emergency tracheal intubation increases oxygen reserves and duration of apnea without desaturation (DAWD) in the hopes of delaying the onset of arterial desaturation , thus prevents hypoxia during a planned or unexpected period of apnea . Side effects of preoxygenation such as stomach inflation should be prevented. Traditional manner of preoxygenation (bag-valve-mask ventilation) is less effective at reducing arterial oxyhemoglobin desaturation than other methods. Lacking of adequate clinical evidences, those effective metoods of preoxygenation weren’t widely used in clinical practice Trend In order to explore a optimal preoxygenation method in ICU, further clinical studies is needed.