Although standard oxygen is the first-line therapy in patients with acute hypoxemic respiratory failure, high-flow nasal oxygen has gained major popularity in ICUs due to its simplicity of application, good comfort for patients, and efficiency in improving oxygenation. Physiological effects of high-flow oxygen therapy can limit the physiological consequences of acute hypoxemic respiratory failure and may mitigate the deleterious effects of high and prolonged inspiratory efforts generated by patients. Although clinical studies have reported a decreased risk of intubation with high-flow oxygen therapy as compared with conventional oxygen therapy, its benefits with regard to survival are uncertain. However, a more precise definition of acute hypoxemic respiratory failure including a classification of severity levels based on oxygenation levels is needed, the objective being to better compare the efficiency of different non-invasive oxygenation supports (conventional oxygen therapy, high-flow oxygen and non-invasive ventilation). Moreover, other clinical trials are needed to confirm the place and the benefit of these oxygenation supports, particularly high-flow nasal oxygen therapy, in acute hypoxemic respiratory failure, especially in the severe forms.