While the hallmark of acute respiratory failure is the inability to maintain gas exchange, no universally accepted definition exists due to the many causes and variable presentations. Traditional definitions rely on arbitrary blood-gas values and may not account for pre-existing cardiopulmonary disease. More recent descriptions incorporate clinical findings such as tachypnea, cyanosis, and use of accessory muscles but may not fully appreciate the spectrum of presentations. For the purposes of this document, we favor a more inclusive definition of acute respiratory failure: a significant change in a patient’s baseline gas-exchange status (given the constellation of available clinical data), which occurs relatively suddenly (usually hours to days) and is potentially life-threatening but which does not require emergent intubation. The causes of respiratory failure are quite diverse and include pneumonia, congestive heart failure, chronic obstructive pulmonary disease (COPD), asthma, pulmonary embolism, and neuromuscular disease, among others