<p>Disorders of water and electrolyte balance are common in intensive care medicine and are prognostically relevant. In the intensive care unit (ICU) setting, the underlying causes are often multifactorial and are substantially influenced by reduced kidney function, iatrogenic fluid and electrolyte administration and organ support treatment. This article provides a&#xa0;practice-oriented overview of the diagnostics and management of hyponatremia and hypernatremia as well as disturbances of potassium, magnesium and phosphate balance. In hyponatremia, symptom severity and the temporal course determine the need for acute treatment. The diagnostic evaluation should be primarily based on laboratory results and rely on serum and urine osmolality as well as urine sodium, as the clinical assessment of volume status is often unreliable. Most cases of hypotonic hyponatremia are caused by dysregulation of the arginine-vasopressin axis, particularly in syndrome of inappropriate antidiuresis (SIAD) or conditions with reduced effective circulating volume. Hypernatremia in the intensive care unit is frequently iatrogenic and associated with increased mortality, with urine osmolality guiding further diagnostic and treatment decisions. Potassium, magnesium, and phosphate disturbances are common and require an early cause-directed management and close monitoring to prevent severe complications.</p>