Application of positive end-expiratory pressure (PEEP) in acute lung injury patients under mechanical ventilation improves oxygenation and increases lung volume. The effect of PEEP is to recruit lung tissue in patients with diffuse lung edema. This effect is particularly important in patients ventilated with low tidal volumes. Measurement of respiratory system mechanics in patients with acute respiratory distress syndrome is important to assess the status of the disease and to choose appropriate ventilator settings that provide maximum alveolar recruitment while avoiding overdistention. In patients with acute respiratory distress syndrome in whom the lungs have been near-optimally recruited by PEEP and tidal volume, the use of recruitment maneuvers as adjuncts to mechanical ventilation remains controversial. The application of PEEP in patients with unilateral lung disease may be detrimental if PEEP hyperinflates normal lung regions, thus directing blood flow to diseased lung regions. In patients with air flow limitation and lung hyperinflation, the application of additional external PEEP to compensate for intrinsic PEEP and flow limitation frequently decreases the inspiratory effort to initiate an assisted breath, thus decreasing breathing work load.