Emergency physicians are required to care for unstable patients with life-threatening conditions, and thus must make decisions that are both quick and precise about unclear clinical situations. There is increasing consensus in favor of using ultrasound as a real-time bedside clinical tool for clinicians in emergency settings alongside the irreplaceable use of historical and physical examinations. B-mode sonography is an old technology that was first proposed for medical applications more than 50 years ago. Its application in the diagnosis of thoracic diseases has always been considered limited, due to the presence of air in the lung and the presence of the bones of the thoracic cage, which prevent the progression of the ultrasound beam. However, the close relationship between air and water in the lungs causes a variety of artifacts on ultrasounds. At the bedside, thoracic ultrasound is based primarily on the analysis of these artifacts, with the aim of improving accuracy and safety in the diagnosis and therapy of the various varieties of pulmonary pathologic diseases which are predominantly "water-rich" or "air-rich". The indications, contraindications, advantages, disadvantages, and techniques of thoracic ultrasound and its related procedures are analyzed in the present review. Core tip: The close relationship between air and water in the lungs causes a variety of artifacts on ultrasounds. At the bedside, thoracic ultrasound is based primarily on the analysis of these artifacts, with the aim of improving accuracy and safety in the diagnosis and therapy of Liccardo B et al . Thoracic ultrasound in ICU the various varieties of pulmonary pathologic diseases which are predominantly "water-rich" or "air-rich". The indications, contraindications, advantages, disadvantages, and techniques of thoracic ultrasound and its related procedures are analyzed in the present review.