Reverse genetics is the creation of a virus from a full-length cDNA copy of the viral genome, referred to as an "infectious clone," and is one of the most powerful genetic tools in modern virology. Since its development in 1999, plasmid-based reverse genetics has been effectively applied to numerous aspects of influenza studies which include revolutionizing the production of seasonal and pandemic influenza vaccine seed strains. Although continual improvement in reverse genetics system is being made in different laboratories for the efficient rescue of the influenza virus, the basic concept of synthesizing viral RNA using RNA polymerase I remains the same. Coupled with in vitro mutagenesis, reverse genetics can be applied widely to accelerate progress in understanding the influenza virus life cycle, the generation of customized vaccine seed strains, development of live-attenuated vaccines, and the use of influenza virus as vaccine and gene delivery vectors.