The pleural space is considered a potential space defined by visceral pleural lining of the lung and parietal pleural lining of the chest wall, diaphragm, and mediastinum. Under normal conditions, the pleural space in the human is a sealed cavity that contains a small volume of fluid estimated to be 0.26 mL/kg body mass, or 15 to 20 mL. The prevalence of pneumothorax in ICU patients requiring mechanical ventilation ranges from 4% to 15%, and incidence in the 1990s was up to 24%, but more recent data show that the incidence has come down to about 3%. This is likely from a change in medical management of ICU patients over time. Thoracic ultrasound use has been rapidly expanding and becoming an essential part of ICU care and emergency medicine. Point-of-care ultrasound is easy to use, readily available in most ICUs, efficient, reliable, and cost effective, with the added advantage of real time imaging. It also has the advantage of safety, as it does not expose the patient to radiation like a chest X-ray or CT scan. Since these exams can be done at bedside, they avoid the risks associated with transporting a critically ill patient. Thoracic ultrasound has also been noted to be helpful in detecting occult pneumothorax. It is shown that comparing ultrasonography to CT scan and chest radiography in diagnosing occult pneumothorax showed 92% diagnostic yield with thoracic ultrasound when compared to 52% diagnostic yield with chest radiograph and when compared with CT in which all the patients were noted to have pneumothorax. Tension pneumothorax is a medical emergency in which there is expanding air volume within the chest cavity with clinical signs and symptoms of progressive hypoxemia, tachycardia, respiratory distress and hypotension, and requires rapid decompression of air from the chest cavity. Presentation can be more pronounced, and progression can be rapid in mechanically ventilated patients when compared to those breathing unassisted. Hence, it is important to make a timely diagnosis of tension pneumothorax and intervene immediately. In critically ill patients, the diagnosis often needs to be made clinically, as there is not enough time to get a chest radiograph for confirmation. Chest ultrasound can be helpful if readily available, but intervention should not be delayed if there is high clinical suspicion for tension pneumothorax.