The baculovirus expression system employingAutagrapha californica nuclear polyhidrosis virus andSpodoptera frugiperda insect cells in culture has proved very popular for high level expression of heterologous genes: In this system, transcription of the foreign gene is usually driven by the hyperactive and temporally regulated polyhedrin gene promoter. Replacement of the polyhedrin gene, which encodes a 29-kDa occlusion protein (non-essential for viral replication), with a gene of interest leads to an occlusion negative phenotype which serves as a visual marker to select for recombinant viruses. Simultaneous expression of multiple genes can also be achieved. The heterologous proteins synthesized in this system are antigenically, immunologically and functionally identical in most respects to their native counterparts. This mini-review will aim at summarizing the potentials and utility of the baculovirus expression vector system and will address some important questions relating to the biology of this system.