The analysis of Pseudorabies (Aujeszky's disease) virus (PrV) at the molecular level has not only considerably increased the knowledge of PrV genes and gene products but has also had a major impact on the development of new vaccines and eradication strategies. So far, ca. 40 PrV genes have been sequenced which constitute approx. 60% of the PrV genome. Analysis of attenuated live vaccine viruses resulted in the identification of gene products that determine viral virulence and neurotropism which include virus-encoded enzymes, membrane glycoproteins, and protein(s) involved in virus particle formation. Based on these results the first genetically engineered live viral vaccine licensed for use is represented by an attenuated PrV mutant. Studies on viral surface glycoproteins led to the elucidation of some of their functions during the infectious cycle proving their role in attachment, penetration, and release of virions. The finding that some of the glycoproteins are nonessential for viral replication and can be deleted from the virus provided the basis for new eradication programs based on differentiation between infected and vaccinated animals. The recent isolation of PrV mutants unable to spread between animals opens up new ways for construction of safe PrV-based vector vaccines. Taken together, PrV can serve as an excellent example where molecular biological research has a direct impact on the practical aspects of controlling, and eventually eradicating an important animal disease.