In recent years, there has been a significant increase in the use of lung ultrasound in critical care, as its value in the assessment of acute respiratory failure has been acknowledged. When performed by an experienced clinician, it enables rapid, accurate, and radiation-free evaluation of a range of conditions directly at the patient's bedside. Lung ultrasound is straightforward to execute and offers greater sensitivity and specificity than commonly used diagnostic methods (eg, chest radiography) for many pathologies. This article describes how to perform lung ultrasound in critically ill patients, discusses key pathologic findings, and describes its use to guide thoracocentesis.