Current efforts to develop vaccines, particularly for aquacultured species, have turned largely to biotechnology because it provides the means to inexpensively produce sufficient quantities of the immunoprotective antigen. These efforts have resulted in several prototype vaccines for fish and the publication of a large number of articles on the subject. However, there are only a few recombinant DNA-based vaccines for aquaculture in the licensing pipeline. Continued funding of research on recombinant DNA vaccines comes from the recognition by industry and government funding agencies that this research can lead to an increased understanding of the mechanisms in protective immunity. This is especially important for fish and shellfish species since our knowledge of the immune mechanisms in these animals is pitifully meagre. This presentation discusses the relative merits of the different recombinant DNA technologies that have been used to produce viral vaccines for fish and the promising approaches that are under consideration to increase the efficacy of these vaccines. There are many approaches to antigen production by recombinant DNA techniques including: (i) the preparation of purified antigenic proteins produced from the cloned viral genes in a variety of vector/host expression systems, (ii) chemical synthesis or the use of fusion vectors to produce peptides corresponding to known epitopes, (iii) defined attenuations, i.e. specific genetic alterations, of live virus vaccines, (iv) the use of live bacterial or viral vectors to deliver resistance genes or viral antigens, (v) anti-idiotype antibodies, and (vi) DNA vaccines where purified plasmid DNA expressing the pathogen gene under a eucaryotic promoter is injected. All of these technologies have been used more or less successfully in the development of vaccines for aquacultured species. However, the requirements for safety, effectiveness, ease of application and low cost/dose restrict their commercial development for aquaculture. The ideal viral vaccine for aquaculture must be effective in preventing death, be inexpensive to produce and license, provide immunity of long duration, and be easily administered. In addition, these vaccines must not only provide protection against the lethal effects of virus infection but prevent the formation of virus persistence. This is especially true for infectious haematopoietic necrosis virus (IHNV) which has been shown to persist in survivors in the presence of high antibody levels. Since resolution of virus persistence is thought to be correlate with cell-mediated immunity, vaccines designed to augment the cell-mediated immunity must be developed for fish. Approaches that are being considered include the use of cytokines in combination with subunit vaccines and the use of specific MHC-I inducer adjuvants with the vaccine. The "tailoring" of vaccine immunogenicity using different combinations of antigen and adjuvant will be presented.