Herpes simplex encephalitis (HSE) is a life-threatening consequence of herpes simplex virus (HSV) infection of the central nervous system (CNS). Although HSE is rare, mortality rates reach 70% in the absence of therapy and only a minority of individuals return to normal function. Antiviral therapy is most effective when started early, necessitating prompt diagnosis. The International Herpes Management Forum (IHMF) has issued guidelines to aid the diagnosis and treatment of HSE. Polymerase chain reaction (PCR) of the cerebrospinal fluid (CSF) is the diagnostic method of choice for HSE, but negative results need to be interpreted in the context of the patient's clinical presentation and the timing of the CSF sampling. CSF virus culture is of little value in all but patients under the age of 6 months. CSF (intrathecal) antibody measurements are not recommended for acute diagnostic purposes. However, demonstration of an intrathecal HSV antibody response may be helpful in retrospective diagnosis or in cases in which CSF is sampled only late after onset of infection and PCR is negative. Serum HSV antibody measurements are not of utility in the diagnosis of HSV encephalitis in adults. In children and young adults, HSV serology may help define whether HSE is part of a primary or a reactivated HSV infection, although the clinical features, therapy, and prognosis of these two forms of HSV encephalitis are similar. The IHMF recommends that all patients with HSE receive intravenous aciclovir 10 mg/kg every 8 h for 14-21 days. Owing to the life-threatening nature of the disease, if there is a delay in diagnostic test results therapy should not be withheld until they become available. After completion of therapy, PCR of the CSF can confirm the elimination of replicating virus, aiding further management of the patient. Clinical trials of other antiviral agents (i.e. adjunctive oral valaciclovir after intravenous aciclovir) for the treatment of HSE are underway. Herpes infection of the CNS, especially with HSV-2, can also cause both monophasic and recurrent aseptic meningitis, as well as myelitis or radiculitis. Limited evidence suggests that aciclovir may be effective in its treatment. Recurrent aseptic meningitis is predominantly caused by HSV-2 infection, and is characterized by self-limited episodes of fever, meningismus and severe headache. Many cases are indistinguishable from cases previously classified as "Mollaret's meningitis", a term that should now be reserved for idiopathic cases of recurrent aseptic meningitis.