Hyperosmolar hyperglycemic state (HHS) is an underrecognized diabetic emergency with high morbidity and mortality. Many features of HHS overlap with those of diabetic ketoacidosis but key differentiators for HHS are serum osmolality greater than 320 mOsm/kg, lack of metabolic acidosis, and minimal to no presence of ketones. HHS is often triggered by an underlying illness—most commonly infection but may also be triggered by stroke, acute coronary syndrome, and other acute illnesses. Treatment guidelines recommend aggressive volume-repletion of osmotic losses in addition to insulin therapy, plus treatment of the underlying cause.