Clinical and experimental research on the effects of positive end-expiratory pressure (PEEP) has produced a plethora of information during the last two decades. The application of PEEP is expected to increase PaO2; however, it is generally agreed that simply using increased PaO2 as the end point is inappropriate. Four mechanisms have been proposed to explain the improved pulmonary function and gas exchange with PEEP: 1) increased functional residual capacity; 2) alveolar recruitment; 3) redistribution of extravascular lung water; and 4) improved ventilation-perfusion matching. The optimal method of applying PEEP is still controversial. The main effect of augmenting PEEP is maintain recruitment of alveolar units that were previously collapsed. Thus, since tidal volume is distributed to more alveoli, peak airway pressure is reduced and compliance is increased. During acute lung injury, and depending on the severity of lung disease, PEEP can markedly alter the compliance of the lung by alveolar recruitment. The greater the alveolar collapse and pulmonary edema, the more the compliance curve of the respiratory system shifts downward and to the right. As PEEP is applied and alveoli recruited, the pressure-volume curve shifts upward and to the left. Despite its intuitive benefit, there were very few controlled studies of the effects of PEEP on ARDS outcome and no prospective randomised controlled trial of PEEP has been ever carried out in patients with acute lung injury and/ or ARDS to evaluate its efficacy until recently.