The high prevalence of herpes simplex virus infections in many communities, its numerous serious physical and psychological complications and its importance in enhancing the transmission of HIV make this virus an obvious target for prevention by vaccination. Randomised clinical trials of only one genital herpes vaccine has shown efficacy so far. Analysis of clinical results is complicated by the difference between disease and infection, different results for males and females and the interaction between HSV-1 and HSV-2 immunity.