Hyperglycaemia is an abnormally high level of glucose in the blood — the defining feature of diabetes mellitus. When insulin is lacking (type 1) or ineffective (type 2), glucose cannot enter cells and accumulates in the blood, typically above 7 mmol/L fasting or 11 mmol/L after meals. Acutely it causes thirst, frequent urination, and fatigue; chronically — the real danger — it damages blood vessels, nerves, kidneys, eyes, and heart, driving the blindness, amputations, kidney failure, and heart attacks of uncontrolled diabetes. Management is the whole discipline of diabetes care: medication, diet, activity, and monitoring aimed at keeping glucose in range. Hyperglycaemia is the slow poison that makes diabetes a disease of vigilance.