Since the invention of PCR by Michael Smith and Kary Mullis, the last two decades have seen an explosion of PCR application in various aspects of biological and medical sciences. Due to its high sensitivity, versatility and reproducibility, PCR has become one of the standard procedures in diagnosis of almost all viral diseases in veterinary medicine. Unlike serological methods, which rely on the presence of specific antibodies, and may lead to false positive or false negative results, PCR detects the presence or absence of the pathogen, thereby providing a better measure of the viruses. Furthermore, with the advent of real-time PCR, researchers now can quantify the amount of virus that is present at different sites in the animal, thereby gaining the ability to determine the stage of infection. Variations of PCRs also allow phenotypic characterization between different viral isolates and between wild-type viruses and vaccines, while allowing simultaneous diagnosis of multiple viruses. PCR has become one of the most commonly used methods in diagnosis of viral disease in livestock and companion animals, and with the development of automated technologies and multiplex PCR systems, which vastly elevate the throughput of PCR assays, increased use of PCR-based techniques is expected in the future.