Hyperglycaemia is a defining feature of diabetes mellitus. It involves an elevated level of glucose in the blood, which develops as a result of the body’s inability to produce insulin or process insulin effectively. If left unchecked and untreated, patients with diabetes are at risk of short-term, potentially life-threatening hyperglycaemic crises such as diabetic ketoacidosis or hyperosmolar hyperglycaemic state. Nurses frequently care for patients diagnosed with diabetes in various clinical settings; therefore, it is essential that they have an awareness of the prevention and management of hyperglycaemia and hyperglycaemic crises. This article explains the causes and clinical manifestations of hyperglycaemic crises, and details the management of patients with these conditions, in accordance with national guidelines.