Haemophagocytic lymphohistiocytosis (HLH) describes a clinical syndrome of hyperinflammation resulting in an uncontrolled and ineffective immune response. It may develop subsequent to a number of recognised genetic mutations or in association with infection, malignancy, autoinflammatory or metabolic conditions. Even with the published diagnostic criteria it can be difficult to make the diagnosis of HLH. Patients presenting acutely to the general paediatrician or paediatric intensivist with a clinical picture of likely sepsis, ie fever, laboratory evidence of inflammatory response, coagulopathy and thrombocytopaenia should be appropriately investigated and managed for sepsis, but the possible diagnosis of HLH should be borne in mind, particularly in the child who deteriorates despite maximal therapy. This review discusses current knowledge on the classification, diagnosis and management of primary and secondary HLH, and suggests a pathway of investigation for the paediatrician faced with a potential case.