Vaccination is a powerful weapon in the control of animal diseases and many highly successful vaccines have been developed, particularly for virus diseases such as rinderpest, foot-and-mouth disease and Newcastle disease. Despite their extraordinary success, however, there are sufficient problems associated with their production and quality control to warrant a re-examination of the methods in current use. Dissection of virus particles into biologically active fragments has shown that their immunizing activity is usually carried on a single protein. With the identification of the genes coding for these individual proteins it is now possible to express these immunogens in both prokaryotic and eukaryotic cells. Moreover, the immunogenic sites of some of these proteins have been identified, synthesized in E. coli cells or by chemical methods and shown to possess immunizing activity. It is too early to put a time-scale on the commercial availability of the new vaccines. However, the potential advantages of such products over conventional vaccines has led to considerable effort in this field of research and progress has been so rapid that new vaccines could be available within the next few years.