In the past ten years there has been enormous progress in the development of eukaryotic viral vectors. In general, these vectors have been developed for one of three reasons: to achieve high levels of expression of a particular gene product (poxvirus, baculovirus, and adenovirus), to clone eukaryotic genes in combination with functional assays (Epstein-Barr virus), of for use as delivery vehicles for the stable introduction of foreign genes into mammalian cells (retroviruses, Epstein-Barr virus, and adeno-associated virus). Each vector has its strengths and weaknesses that are rooted in the sometimes bewildering strategies that the parent viruses use for propagation. No one of these vectors is appropriate for all of the problems that a molecular biology laboratory is likely to encounter, and few of us are knowledgeable in the molecular virology of all of these viruses. This volume represents an attempt by the authors to assemble a review of these vectors in one place and in a form useful to laboratories that do not necessarily have experience with eukaryotic viruses. Clearly, any virus can be modified to serve as a vector for some purposes, and it was not possible to include a description of all of these. In addition, one eukaryotic vector, SV40 (the first one developed), has been reviewed so widely that we saw no reason to include it here.