The present invention provides a library of viral vectors, wherein each member comprises a first heterologous DNA encoding a first gene product and a second heterologous DNA encoding a second gene product. The first heterologous DNA is common to each member of the library, while the second heterologous DNA varies between members of the library. The present invention additionally provides a method of constructing a library of viral vectors. The method comprises carrying out homologous recombination between a first DNA molecule and a second DNA molecule to form a pool of intermediate viral vector genomes. One or more linear third DNA molecules are ligated into the pool of intermediate viral genomes to produce a library of viral vector genomes. Alternatively, homologous recombination between linear DNA molecules and recipient DNA molecules produces a library of viral vector genomes. The library of viral vector genomes is converted into a library of viral vectors.