Vaccines against Aujeszky's disease are not only used to prevent the clinical consequences of a field infection, but also to support eradication of the virus. The current Aujeszky's disease vaccines (ADV) protect against severe clinical signs of disease and they reduce but usually do not prevent virus multiplication and excretion or the establishment of latency after infection. Vaccines also limit virus multiplication after reactivation. The efficacy of vaccination is reduced by passively acquired maternal antibodies. The mechanisms that afford immunity to the virus are only poorly understood. No simple parameters for immunity are therefore available. The European Pharmacopoeia formulates requirements for inactivated and live ADV vaccines for parenteral use. The vaccines must be safe; they must not induce local or systemic reactions; they must not be transmitted to unvaccinated pigs; they must not be transmitted by semen and across the placenta; the attenuation must be irreversible (live vaccines); the inactivation must be complete (inactivated vaccines); they must prevent mortality and limit growth retardation after challenge infection; the vaccine must not contain contaminating micro-organisms and viruses. No requirements have been formulated with regard to reduction of excretion of challenge virus after experimental infection, efficacy in pigs with maternal antibodies, reproducibility of efficacy studies, reduction of virus transmission under field conditions, the presence of a serological marker, safety for other species, and safety and efficacy of intranasally administered vaccines. Future developments should be directed to the development and evaluation of ADV vaccines that are able to limit transmission of the virus.