Nucleic acid immunisation entails the delivery of DNA (or RNA) encoding a vaccine antigen to the recipient. The DNA is taken up by host cells and transcribed to mRNA, from which the vaccine proteins are then translated. The expressed proteins are recognised as foreign by the host immune system and elicit an immune response, which may have both cell-mediated and humoral components. DNA vaccines offer a number of advantages over conventional vaccines, including ease of production, stability and cost. They also allow the production of vaccines against organisms which are difficult or dangerous to culture in the laboratory. This review describes the principles of DNA vaccination and the application of DNA vaccines to veterinary species. Although a great deal of developmental work is required before the technology can give rise to commercial vaccines in domestic animals, there is ongoing research in many fields and it is expected that a number of exciting developments will arise in the next decade.