Mechanical ventilation is frequently initiated by emergency physicians. Further, the physician on duty in the emergency department is frequently responsible for evaluating ventilated patients who decompensate in the intensive care unit when other physicians are not present in the hospital. A bewildering array of features on new mechanical ventilators has made their appropriate and effective use increasingly complex. Knowledge of the pathophysiology of acute respiratory failure and changes in lung physiology during positive pressure ventilation will aid the emergency physician in choosing an appropriate ventilator modality and initial settings to maximally benefit patients with respiratory insufficiency due to various causes. An appreciation of the adverse effects of mechanical ventilation and problems commonly encountered in patients on ventilators will prepare the emergency physician to rapidly assess and effectively manage the patient who deteriorates in this setting.