Diabetic ketoacidosis (DKA) and hyperglycemic hyperosmolar state (HHS) are the most serious and life-threatening hyperglycemic emergencies in diabetes. DKA is more common in young people with type 1 diabetes and HHS in adult and elderly patients with type 2 diabetes. Features of the 2 disorders with ketoacidosis and hyperosmolality may coexist. Both are characterized by insulinopenia and severe hyperglycemia. Early diagnosis and management are paramount. Treatment is aggressive rehydration, insulin therapy, electrolyte replacement, and treatment of underlying precipitating events. This article reviews the epidemiology, pathogenesis, diagnosis, and management of hyperglycemic emergencies.