There are three main reasons for wishing to modify baculovirus genomes. The most common arises from the use of baculoviruses to express foreign genes. Baculovirus expression systems frequently give high level expression of proteins with full biological activity. This combination of highly desirable features is responsible for the widespread popularity of baculovirus expression systems. Second, is the renewed interest in the use of baculoviruses as biological insecticides. Wildtype baculoviruses are highly specific insecticides with no toxicity for non-target organisms and have been used to control insect pests for many years (Granados & Federici, 1986). Recently, considerable effort has been made to enhance the insecticidal potency of baculoviruses by introducing foreign genes, such as hormones or toxins, or making other modifications to the viral genome (for reviews see: Wood & Granados, 1991; Bonning & Hammock, 1992). The third reason is that the ability to introduce mutations and other modifications into a baculovirus genome provides a powerful tool for studying baculovirus biology. Studying the complex interactions between baculoviruses and their hosts, both at the cellular and organismal levels, may provide valuable insights into other viral infections. In addition, greater understanding of baculovirus biology is vital for the continued improvement of baculovirus expression systems and baculovirus insecticides. The large size of baculoviral genomes, 80–200 kb, makes it difficult to manipulate the viral DNA directly due to its sensitivity to shearing and to the paucity of unique restriction sites. The original method for introducing mutations or foreign genes into a baculovirus circumvented these difficulties using a twostep approach; a segment of the viral DNA was cloned into a plasmid vector, modified, and then homologous recombination inside insect cells was used to transfer the modification back into the viral genome (Smith et al., 1983; Pennock et al., 1984). Many recombinant viruses have been made using this method; however, it is inefficient and time consuming. As the need for recombinant viruses grew, more efficient methods were developed. This review will describe the major methods that have been developed to construct recombinant baculoviruses and their relative advantages and weaknesses (for an earlier review see Davies, 1994). The various methods are grouped according to the host in which the recombinant is generated: insect cells, heterologous hosts, and in vitro. Most methods for generating recombinants have been developed using AcMNPV, therefore this review will concentrate on this virus.