Diabetic ketoacidosis (DKA), hyperglycemic hyperosmolar state (HHS), lactic acidosis (LA), and hypoglycemia are acute and potentially life-threatening complications of diabetes. DKA and severe hypoglycemia are more common in type 1 diabetes, while HHS without ketoacidosis is associated more frequently with type 2 diabetes. In the United States, the SEARCH for Diabetes in Youth study reported that 29% of patients age <20 years with type 1 diabetes and 10% with type 2 diabetes presented in DKA at diagnosis. The frequency of DKA among adult patients at diagnosis is unknown. A small group of high-risk patients accounts for most recurring DKA in longstanding type 1 diabetes, but the incidence remains high—approximately 1–12 episodes per 100 patient-years. Deaths in the United States with DKA listed as the underlying cause during 2000–2009 decreased 35%, from an annual rate of 12.9 per 100,000 people with diabetes in 2000 and 2001 to 8.4 per 100,000 people with diabetes in 2009. Estimated rates of hospital admissions for HHS are lower compared to DKA. HHS accounted for <1% of all admissions related to diabetes. HHS remains uncommon, but recognition of the state has increased, partially because of high case fatality, exceeding 20% in some patient groups. In 2001–2010, LA accounted for 1.2% of all hospitalizations in diabetic patients. Also in 2001–2010, hypoglycemia was listed as an underlying cause in nearly 288,000 hospitalizations, which represented 5.4% of total hospitalizations due to diabetes. Severe hypoglycemia, i.e., coma or seizure secondary to diabetes treatment, remains high (up to five episodes per 10 patient-years) and has increased among patients who aim for lower glycosylated hemoglobin (A1c) targets without appropriate initial education and ongoing support. All four acute complications are theoretically preventable; unfortunately, they still account for enormous morbidity, hospitalizations, and mortality among diabetic patients and contribute significantly to the high costs of diabetes care.