The incidence of pleural effusions in the intensive care unit varies depending on the screening methods, from approximately 8% for physical examination to more than 60% for routine ultrasonography. Several factors contribute to the occurrence of pleural effusions in intensive care unit patients: large amounts of intravenous fluid are often administered, pneumonia is common, and heart failure, atelectasis, extravascular catheter migration, hypoalbuminemia, or liver disease are present in many intensive care unit patients. In surgical intensive care units, cardiac or abdominal surgery is often followed by pleural effusions, and in trauma patients, hemothorax is a dreaded event. Because no clinical parameter excludes pleural infection, and because of the impact of thoracentesis on diagnosis and treatment, this procedure should be performed unless contraindicated. Thoracentesis is safe in mechanically ventilated patients. The author discusses the following points regarding pleural effusions in the intensive care unit: screening intensive care unit patients for pleural effusion, safety of thoracentesis in patients receiving invasive mechanical ventilation, distinguishing exudates from transudates, and diagnosing and managing infected pleural effusions in critically ill patients. Lastly, the author suggests a research agenda for pleural effusions in intensive care unit patients.