Non invasive ventilation is defined as mechanical ventilation without the use of endotracheal intubation and has been increasingly established within intensive care units during the last decades. Negative pressure ventilation and non invasive positive pressure ventilation have been successfully applied, first in chronic respiratory failure (CRF) due to various causes and later in acute respiratory failure (ARF). In this review ventilation modes, indications, contraindications and side effects of non invasive ventilation are analysed and the impact of non invasive ventilation on the physiology, pathophysiology and outcome of CRF and ARF, and possible applications in CRF (restrictive chest and pulmonary diseases, neuromuscular diseases and COPD) and ARF are discussed. It is concluded that non invasive ventilation should be included in the routine management of respiratory failure at all intensive care units.