Acid-base disorders in the intensive care unit are frequent and manifold. Respiratory disturbances are caused by pulmonary and central nervous diseases; metabolic disturbances are caused by not only gastrointestinal and renal diseases but also hypoxic and septic situations. As an initial step to differentiate between the different acid-base disorders, the parameters pH, pCO2, bicarbonate concentration, and the anion gap are essential. For further evaluation, additional parameters are necessary. A rapid classification of the disorder is mandatory for an immediate and effective therapy which mainly depends on eliminating the underlying cause but also includes the differential implementation of mechanical respiration, volume, electrolyte and buffer therapy, as well as specific renal replacement therapy. As a result, the prognosis of critically ill patients in intensive care units will improve.