Herpes simplex virus (HSV) infection can cause significant morbidity and mortality in neonates (age less than 29 days) and occurs in 0.2% to 0.5% of neonates undergoing evaluation for serious bacterial illness in the emergency department. This article reviews the clinical features, diagnosis, and recommended treatment of neonatal HSV. The 3 clinical classifications of neonatal HSV infection are described and include: disseminated, central nervous system, and skin, eye, or mouth diseases. A comprehensive medical evaluation is described for the neonate with suspected HSV, including tests to identify the virus and to classify neonates accurately, because treatment and prognosis differ between disease classifications. Also presented are current recommendations for treatment of acute illness with parenteral acyclovir, as well as long-term oral acyclovir after acute illness. Current evidence and expert opinions are presented regarding the initiation of empiric HSV treatment for neonates undergoing serious bacterial illness evaluation.