The importance of monitoring and evaluating electrolyte and water disturbances in neurosurgical patients cannot be understated because they can lead to brain injury, seizures and cerebral oedema. They occur as a result of a underlying disease, raised intracranial pressure or due to surgical procedure. Most important are sodium and water abnormalities, especially in patients with hypophyseal disorders, traumatic brain injury (TBI) and subarachnoid haemorrhage (SAH). In those patients hypernatremia is mostly caused by diabetes insipidus (DI) and hyponatremia by syndrome of inappropriate antidiuretic hormone secretion (SIADH) or cerebral salt wasting (CSW). Diabetes insipidus is usually relatively straightforward to diagnose and treat, unless it is adipsic diabetes insipidus. With regard to hyponatremia, situation is much less straightforward. The differentiation between syndrome of inappropriate antidiuretic hormone secretion and cerebral salt wasting poses specific diagnostic challenges. Also, correct diagnosis is of significant importance because of fundamentally different approaches in treatment: fluid restriction in former and fluid administration in latter. While neurointensive care patients can experience a variety of electrolyte abnormalities beyond sodium and water, these other electrolyte abnormalities are seen at a frequency similar to the general intensive care unit patient population. They can cause a variety of symptoms, such as: hypotension, arrhythmias, muscle weakness, nerve irritation.