Heterogeneity is a familiar concept to paediatric intensive care physicians, because patients, pathologies, and treatments vary both within the paediatric intensive care unit (PICU) and within a given disease process. Paediatric acute respiratory distress syndrome (ARDS) typifies this problem. Children with ARDS vary in terms of age (and therefore lung development), underlying cause (pneumonia, sepsis, or trauma), pre-existing comorbid conditions, and baseline immunological state. There is not one pathway by which ARDS occurs or manifests, as indicated by the identification of hyperinflammatory and hypoinflammatory subtypes in adults and children.