Acute respiratory distress syndrome (ARDS) is an acute severe lung disease commonly encountered in intensive care units (ICU). It can be caused by several triggers, including pneumonia or trauma. It is characterised by widespread injury of the alveolar–capillary membrane, resulting in protein-rich noncardiogenic pulmonary oedema (fluid accumulation in the lungs) and acute respiratory failure (ARF). ARDS results in severe hypoxaemia, which is refractory to oxygen treatment and requires assisted ventilation. It shares some of the features of infant respiratory distress syndrome (IRDS), which results from insufficient production of surfactant that normally lines the alveoli and reduces the surface tension of the alveolar lining fluid, preventing the collapse of the airspace. In contrast to ARDS, IRDS can be treated successfully using surfactant. ARF, a term sometimes used synonymously with ARDS, is far broader and comprises respiratory failure resulting from many other conditions: for example, chronic obstructive pulmonary disease (COPD). The term ‘acute lung injury’ (ALI) was previously used to characterise a milder form of ARDS, but it is no longer recommended for use.