Mechanical ventilation is commonly used in patients admitted to intensive care units (ICUs). Clinical studies have investigated some of the aspects associated with its utilization, and have contributed to improve the knowledge and management of patients with acute respiratory failure. In 1998, the main researchers conducted the first international study on mechanical ventilation, including 5183 patients who were mechanically ventilated longer than 12 hours. Six years later, the investigators carried out the second international study on mechanical ventilation including 4968 patients. These studies allowed to analyze the evolution of mechanical ventilation and to judge the concordance of practice with results of randomized trials. The main finding of this study was the high degree of concordance between observed changes in mechanical ventilation practice and changes predicted from reports of randomized controlled trials. However, despite apparently beneficial changes in ventilatory practice, the results did not able to detect significant differences in clinical outcomes over this time period. These results have served as an updated benchmark on the usual care and outcomes of mechanically ventilaed patients in a 'real-world' setting. Lastly, in 2010 the main researchers finished the Third International Study of Mechanical Ventilation and were able to find out, for the first time, a significant reduction of ICU mortality in mechanically ventilated patients. Then, the investigators propose to lead up the fourth-international study of mechanical ventilation. This study will keep the same methodology and it will be a prospective, international and multicenter non interventional single-cohort study, which will enroll consecutive patients who have received mechanical ventilation (invasive and noninvasive ventilation) for at least 12 hours during a 1-month period, and will follow each patient for the duration of mechanical ventilation, up to 28 days. Despite of all the current implementations in different ventilatory strategies, and new ventilatory modalities, the mortality and clinical outcomes in critically ill patients under mechanical ventilation is poorly known all over the world. The aims of this study are: