Patients with dyspnea are routinely assessed with physical examination, lung auscultation and chest X-rays. Auscultation and bedside radiography have a low accuracy in the critically ill. Chest X-ray images remain suboptimal in more than one-third of cases and are poorly correlated to lung CT images. Lung ultrasound is invaluable as a bedside diagnostic tool of the dyspneic patient since, in a few minutes, it may help distinguish the typical features of most common causes of dyspnea. The lung has been improperly considered to be poorly accessible using ultrasound, because air prevents the progression of the ultrasound beam with production of reverberation artifacts under the lung surface. This article reviews the scanning technique, and the normal and pathological findings of lung ultrasound, and underscores its utility in providing the emergency physician with a new, fast and reliable diagnostic tool.