Herpes simplex virus (HSV) infections of the central nervous system (CNS) can occur within weeks after birth (neonatal HSV disease) or in childhood or adulthood [herpes simplex encephalitis (HSE)]. Most cases of neonatal HSV disease are caused by HSV type 2, whereas virtually all cases of HSE are caused by HSV type 1. Diagnostic advances made during the past decade include the application of polymerase chain reaction (PCR) technology to cerebrospinal fluid from patients with suspected HSV CNS disease to evaluate for the presence of HSV DNA. Although not foolproof, PCR is a powerful diagnostic tool that has supplanted brain biopsy as the modality of choice for diagnosing HSV CNS disease, in no small part because of the invasiveness of brain biopsy. PCR also can provide information regarding the therapeutic response to antiviral therapy. Efforts made during the past decade to improve the outcome of HSV CNS disease have focused on increased doses of intravenous acyclovir administered for longer durations of time. Although advances have been achieved, morbidity and mortality rates from neonatal HSV disease and HSE remain unacceptably high. © 2003 Elsevier Inc. All rights reserved.