Acute meningitis and encephalitis are medical emergencies that require prompt assessment (usually by cerebral imaging and lumbar puncture) and treatment; specialist consultation is recommended. In acute meningitis, early administration of antibiotics can be life-saving (usually high-dose penicillin and/or a third-generation cephalosporin); antibiotics may be needed before referral to hospital. Emergence of penicillin and cephalosporin resistance in Streptococcus pneumoniae has necessitated more complex antibiotic regimens that include vancomycin or rifampicin for empirical treatment of meningitis. Adjunctive dexamethasone therapy may be of benefit in children with Haemophilus influenzae meningitis; there is no controlled evidence of its benefit in adults, but it could be considered in those with raised intracranial pressure. In possible encephalitis, empirical therapy with intravenous aciclovir should be given to cover herpes simplex virus (HSV) until the cause is established; HSV encephalitis may be fatal and leaves up to 50% of survivors with long-term sequelae.