Non invasive mechanical ventilation represents all the techniques of mechanical ventilation not using endotracheal connection to the respirator: Non invasive mechanical ventilation is essentially represented nowadays by facial (generally used for acute respiratory failure) and nasal ventilation (widely used for long term mechanical ventilation at home). Following in France the first studies of Rideau in the '80 in patients with chronic respiratory failure secondary to muscular dystrophy Meduri, then Brochard demonstrated the interest of non invasive mechanical ventilation in the management of acute on chronic respiratory failure. A simple algorithm is thus able to recommand in first attempt use of non invasive mechanical ventilation in acute on chronic respiratory failure except if the patient presents in a severe respiratory status imposing in first line endotracheal ventilation. In parallel, more recent studies have demonstrated the interest of that technique in immediate ventilatory-link in the spectrum of weaning of endotracheal mechanical ventilation. In hypoxic acute respiratory failure, non invasive mechanical ventilation remains controversial and seems to be better indicated in moderate to severe respiratory status.