This review discusses the usefulness of bedside lung ultrasound in the diagnostic distinction between the various causes of acute dyspnoea in the emergency department, with special attention to the differential diagnosis of pulmonary oedema and exacerbation of chronic obstructive pulmonary disease (COPD). This is made possible by using mid- to low-end scanners and simple acquisition techniques accessible to both radiologists and clinicians. Major advantages include ready availability at the bedside, the absence of ionising radiation, high reproducibility and cost efficiency. The technique is based on the recognition and analysis of sonographic artefacts rather than direct visualisation of the pulmonary structures. These artefacts are caused by the interaction of water-rich structures and air, called comet tails or B-lines. When such artefacts are widely detected on anterolateral transthoracic lung scans, diffuse alveolar-interstitial syndrome can be diagnosed, which is often a sign of acute pulmonary oedema. This condition rules out exacerbation of COPD as the main cause of acute dyspnoea.