High-flow nasal oxygen (HFNO) is commonly used in acute respiratory failure. Acute hypoxemic respiratory failure is defined as an oxygen saturation below 88% or a baseline partial pressure of arterial oxygen to fractional inspired oxygen concentration ratio of less than 200 mm Hg. In HFNO, warm and humidified oxygen is supplied by nasal cannula at a flow rate up to 60 L per minute. Other options for treating respiratory failure include noninvasive ventilation (NIV) with continuous or bilevel positive airway pressure ventilation and conventional oxygen therapy with low-flow nasal oxygen or oxygen delivered by face mask. The American College of Physicians published guidelines for use of HFNO in adults based on a systematic review focusing on all-cause mortality, hospital length of stay, hospital-acquired pneumonia, intubation/reintubation, intensive care unit admissions, and patient comfort.