Noninvasive mechanical ventilation (NMV) now represents the first step in the management of acute on chronic respiratory failure (A/CRF). During the last 5 yrs, many studies have confirmed the feasibility of NMV in an acute setting, either by facial or nasal interface, used in addition to volumetric or barometric respirators, to manage A/CRF. The best indications for NMV are slowly progressive A/CRF, frequently represented by chronic obstructive pulmonary disease (COPD), or restrictive pulmonary disease. The criteria to initiate NMV in such patients are worsening of respiratory status and arterial blood gas (ABG) values, with increased hypoxia, hypercapnia and respiratory acidosis, despite optimal management with medication, physiotherapy and oxygen therapy. Respiratory encephalopathy is not an absolute contraindication; however, bronchial hypersecretion indicates that care is needed under NMV. Invasive mechanical ventilation with endotracheal (ET) intubation is discussed in the case of failure of NMV, when clinical status and ABG values worsen in spite of it. The signal for ET intubation is then obvious, represented by severe dyspnoea leading to respiratory pauses or arrest, severe cyanosis, and signs of haemodynamic instability. Despite immediate evidence of ominous cardiorespiratory inefficiency, ET intubation may be delayed and often avoided with the help of NMV. Criteria should be studied to identify guidelines for cessation of NMV, in order not to continue with the technique too long considering the safety of the patient. Indications for NMV in other kinds of ARF have received less study and are more controversial.