High Flow Nasal Cannula refers to the delivery of gas intra-nasally via loose-fitting cannulae at flows up to 60 l/min and with FIO2 that can be varied between room air and 100%. The modality is generally well tolerated because the nasal cannulae are comfortable and the gas is heated and humidified to body temperature and saturation. In addition, it doesn’t interfere with speech or eating. Physiologically, it has a number of advantages over standard oxygen, including the humidification that enhances ciliary action and secretion removal, high inspiratory flow that cuts down on entrainment of room air, thus ensuring a more reliable delivery of a targeted FIO2, flushing out of upper airway dead space that improves efficiency of ventilation, reduction of respiratory rate that helps to cut down on work of breathing per minute, and a small amount of positive end expiratory pressure that may help to counterbalance auto-PEEP. In addition, it is clearly better tolerated than either standard mask oxygen delivery systems or noninvasive ventilation.