Oxygen-conserving devices include transtracheal catheters, reservoir cannulas, and demand oxygen delivery systems. They are designed to extend the amount of time portable oxygen cylinders will last and correct hypoxemia with a lower flow of oxygen. Transtracheal catheters increase the fraction of inspired oxygen by delivering oxygen directly to the trachea, bypassing the dead space of the oropharynx and improving the efficiency of the upper airway as a reservoir. Reservoir cannulas increase the fraction of inspired oxygen at the beginning of the inspiratory phase. Demand oxygen delivery systems have a valve that is activated during inspiration, meaning that oxygen is only delivered during this stage of the respiratory cycle. Each system has advantages and disadvantages arising from differing design features. Prescription should be based on individual tests in all cases to ensure optimal oxygen delivery during rest, exercise, and sleep.