Mannitol has replaced other diuretics as the agent of first choice for control of raised intracranial pressure (ICP) after brain injury. Mannitol should be given as a bolus intravenous infusion, over 10 to 30 mins, in doses ranging from 0.25 to 1.0 g/kg body weight. It may be given when high ICP is suspected, prior to computed tomography scanning, e.g., in patients who develop a fixed, dilated pupil or neurologic deterioration. This agent may also be used pre- or intraoperatively in patients with intracranial hematomas, and when high ICP is demonstrated in the ICU. It is more effective and safer when administered in bolus doses than as a continuous infusion. Mannitol may be safely used during the early resuscitation phase in hypovolemic patients with concomitant head injury, provided that plasma expanders and/or crystalloid solutions are given to correct the hypovolemia simultaneously. A Foley catheter should always be inserted when mannitol is used. Serum osmolality should be measured frequently after mannitol and maintained < 320 mOsm to avoid renal failure. Its beneficial effects and the rationale for its use are also reviewed.