Functional expression of recombinant proteins has become a routine, but critical tool in modern molecular biology. Since their introduction, the use of baculovirus vectors to produce proteins for purification has become one of the most widely-used viral gene delivery systems as expression levels obtained are difficult to match with any other eukaryotic expression system. Extensive engineering to simplify and accelerate the process of recombinant virus construction has made this system accessible to virtually any modern biological laboratory. The utility of baculoviruses has been broadened with the discovery that appropriately modified virus can mediate gene expression in a wide variety of mammalian cell lines, and thus can function as a flexible cell-based assay development tool. The wide range of applications and potential for commercialization of products leads to consideration of a number of aspects of the system.