Lung has been considered as the blind spot of ultrasound.While, with the development of ultrasonic technique, lung ultrasound attracts more and more attention, and plays an important role in the diagnosis of lung disease in critically ill patients.This review summarizes the principles and methods of lung ultrasound, including common normal and abnormal signs, the clinical applications of lung ultrasound in critically ill patients (pleural effusion, pneumothorax, atelectasis, acute pulmonary edema, and pulmonary embolism).