Haemophagocytic syndrome, or haemophagocytic lymphohistiocytosis (HLH), is a hyperinflammatory disorder characterised by uncontrolled activation of the immune system. It can result from mutations in multiple genes involved in cytotoxicity or occur secondary to a range of infections, malignancies or autoimmune rheumatic diseases. In the latter case, it is also known as macrophage activation syndrome (MAS). Characteristic features are persistent fever, hepatosplenomegaly, petechial/purpuric rash, progressive cytopenias, coagulopathy, transaminitis, raised C reactive protein, falling erythrocyte sedimentation rate, hypertriglyceridaemia, hypofibrinogenaemia and extreme hyperferritinaemia often associated with multi-organ impairment. Distinguishing HLH from systemic sepsis can present a major challenge. Criteria for diagnosis and classification of HLH and MAS are available and a serum ferritin >10 000 µg/L is strongly supportive of HLH. Without early recognition and appropriate treatment, HLH is almost universally fatal. However, with prompt referral and advancements in treatment over the past two decades, outcomes have greatly improved.