Diabetic ketoacidosis (DKA) and the hyperglycemic hyperosmolar state (HHS) represent potentially life-threatening situations in adults. Therefore, rapid comprehensive diagnostic and therapeutic measures with close monitoring of vital and laboratory parameters are required. The treatment of DKA and HHS is essentially the same and replacement of the mostly substantial fluid deficit with several liters of a physiological crystalloid solution is the first and most important step. Serum potassium concentrations need to be carefully monitored to guide its substitution. Regular insulin or rapid acting insulin analogues can be initially administered as an i.v. bolus followed by continuous infusion. Insulin should be switched to subcutaneous injections only after correction of the acidosis and stable glucose concentrations within an acceptable range.