The value of animal models for assessing the quality of veterinary viral vaccines is not to be despised, particularly since one has access to target animal models which are often more relevant than those in the laboratory, especially for challenge/protection studies. Immune protection involves complex immunological phenomena and processes. It is particularly true whenever cellular immunity plays a crucial role because it is still easier to measure antibody responses than cellular ones in vitro. Nevertheless the trend is to replace animal models by in vitro system whenever possible. The problem of the replacement of in vivo by in vitro models is further impeded in Europe by the necessity to comply with Pharmacopoeia monographs where the use of laboratory and/or target animals is often requested. Recent advances have been made with several inactivated viral vaccines such as equine influenza, where strain variability poses a special problem, or rabies, for which the use of inactivated instead of attenuated vaccines for vaccination of animals became compulsory in many countries.