It is well established that general anesthesia, with or without paralysis, causes profound changes in respiratory function. From a clinical point of view, the more important consequence of this impairment is a decreased efficiency of gas exchange, with a decreased blood oxygenation. The main reason of this respiratory embarrassment is the intraoperative occurrence of atelectasis, mainly in the dependent lung regions. The amount of atelectasis, computed through Computerized Tomography, correlates with the amount of intrapulmonary shunt; thus, alveolar collapse and ventilation/perfusion mismatching are considered the most important factors for poor respiratory function. This deterioration seems also to play a crucial role in obese patients, who have poorer respiratory function and gas exchange than normal subjects already in physiological conditions. Different ventilatory approaches have been tried to resolve and eventually prevent the anesthesia-induced atelectasis. In normal subjects, the sole application of positive end-expiratory pressure (PEEP) seems to be an useless tool for improving gas exchange, probably because of changes in hemodynamics functions. The only effective application of PEEP seems to be in association to an alveolar recruitment manoeuvre. As the anesthesia-induced atelectasis are also present in the postoperative period, this ventilatory approach may also be used to prevent this condition. In obese patients PEEP seems to have a major effectiveness than in normal subjects, with an improvement of lung volumes, respiratory mechanics, gas exchange and an occurrence of recruitment. However, further studies are necessary to define optimal value of PEEP and tidal volume for different types of patients.