Herpes simplex virus type 2 (HSV-2) is a suitable target for a vaccine, despite available antiviral therapies, because the virus causes lifelong infection and significant medical and psychosocial morbidity. A vaccine has the potential to reduce HSV acquisition, disease severity and the number of cases of neonatal herpes. It could also reduce transmission of HIV, which is epidemiologically linked to HSV. Prophylactic vaccines for HSV-2 must give broad and durable immunity across all mucosal surfaces to be effective. This is a significant challenge, as the major determinants of effective immunity have not yet been identified.