Non-invasive ventilation (NIV) is the use of breathing support administered through a face mask, nasal mask, or a helmet. Air, usually with added oxygen, is given through the mask under positive pressure; generally the amount of pressure is alternated depending on whether someone is breathing in or out. It is termed "non-invasive" because it is delivered with a mask that is tightly fitted to the face or around the head, but without a need for tracheal intubation (a tube through the mouth into the windpipe). While there are similarities with regard to the interface, NIV is not the same as continuous positive airway pressure (CPAP), which applies a single level of positive airway pressure throughout the whole respiratory cycle; CPAP does not deliver ventilation but is occasionally used in conditions also treated with NIV.

Non-invasive ventilation is used in acute respiratory failure caused by a number of medical conditions, most prominently chronic obstructive pulmonary disease (COPD); numerous studies have shown that appropriate use of NIV reduces the need for invasive ventilation and its complications. Furthermore, it may be used on a long-term basis in people who cannot breathe independently as a result of a chronic condition.

NIV for acute respiratory failure is used particularly for severe exacerbations of chronic obstructive pulmonary disease (COPD) but also for acute decompensated heart failure and other acute conditions. NIV can be used acutely and long-term. In some people who have presented with acute respiratory failure, there is an ongoing need for long-term use of NIV at home.

Non-invasive ventilation has been suggested in the treatment for coronavirus disease 2019 (COVID-19) where shortages of invasive ventilation equipment and facilities may arise. The risk of poorly fitting masks emitting aerosols can require full protection gear for caregivers.

The most common indication for acute non-invasive ventilation is for acute exacerbation of chronic obstructive pulmonary disease. The decision to commence NIV, usually in the emergency department, depends on the initial response to medication (bronchodilators given by nebulizer) and the results of arterial blood gas tests. If after medical therapy the lungs remain unable to clear carbon dioxide from the bloodstream (respiratory acidosis), NIV may be indicated. Many people with COPD have chronically elevated CO₂ levels with metabolic compensation, but NIV is only indicated if the CO₂ is acutely increased to the point that the acidity levels of the blood are increased (pH<7.35). There is no level of acidity above which NIV cannot be started, but more severe acidosis carries a higher risk that NIV alone is not effective and that mechanical ventilation will be required instead.

Bronchiectasis may lead to acute hypercapnic respiratory failure (AHRF), and NIV may be used similarly as for COPD. This is particularly the case where the underlying cause is cystic fibrosis. Cystic fibrosis also causes high volumes of sputum (phlegm) which may require specialised physiotherapy assistance and sometimes the insertion of a mini-tracheostomy to clear this.

In people with chest wall deformity or neuromuscular disease, NIV may be commenced if the CO₂ level is elevated even if it has not yet caused acidosis. In neuromuscular disease, a breathing measurement known as the vital capacity is used to determine a need for breathing support.

Obesity hypoventilation syndrome (OHS) may cause acute hypercapnic respiratory failure. When this is the case, the criteria for commencing acute NIV are similar to those for COPD (decreased pH, elevated CO₂), although there are some scenarios where NIV may be initiated in hospitalized people despite a normal pH; these include people with daytime somnolence, sleep-disordered breathing and/or evidence of right ventricle heart failure.