Diabetic ketoacidosis (DKA), hyperglycaemic hyperosmolar state (HHS) and hypoglycaemia are serious complications of diabetes mellitus that require prompt recognition, diagnosis and treatment. DKA and HHS are characterized by insulinopaenia and severe hyperglycaemia; clinically, these two conditions differ only by the degree of dehydration and the severity of metabolic acidosis. The overall mortality recorded among children and adults with DKA is <1%. Mortality among patients with HHS is ∼10-fold higher than that associated with DKA. The prognosis and outcome of patients with DKA or HHS are determined by the severity of dehydration, the presence of comorbidities and age >60 years. The estimated annual cost of hospital treatment for patients experiencing hyperglycaemic crises in the USA exceeds US$2 billion. Hypoglycaemia is a frequent and serious adverse effect of antidiabetic therapy that is associated with both immediate and delayed adverse clinical outcomes, as well as increased economic costs. Inpatients who develop hypoglycaemia are likely to experience a long duration of hospital stay and increased mortality. This Review describes the clinical presentation, precipitating causes, diagnosis and acute management of these diabetic emergencies, including a discussion of practical strategies for their prevention.