Fluid management in the neurocritical care unit (NCCU) requires careful consideration of the cerebral physiology and implications in acute brain injury conditions. Crystalloids are recommended as the preferred fluid, and using colloids, glucose-containing solutions, other hypotonic solutions, or albumin for maintenance or resuscitation is discouraged in the setting of acute brain injury in the NCCU. Avoiding both hypovolemia and hypervolemia and targeting normovolemia are beneficial and, therefore, suggested. Monitoring with multiple hemodynamic variables, invasive blood pressure, and fluid balance is recommended during fluid management in the NCCU. Fluid therapy may require alterations in specific conditions of sodium and water disturbances, such as cerebral salt wasting, syndrome of inappropriate anti-diuretic hormone secretion, and diabetes insipidus. Osmotherapy is commonly employed after acute brain injury to reduce intracranial pressure and improve cerebral perfusion and oxygenation. The clinical condition should determine the choice of the hyperosmolar agent rather than drug characteristics, as both are equivalent in terms of effectiveness in reducing intracranial pressure and improving neurological outcomes and side-effect profiles.