Currently, there is no single source that permits comparison of the factors, elements, enzymes and/or mechanisms employed by different classes of viruses for genome replication. As a result, we (and our students) often restrict our focus to our particular system, missing out on the opportunity to define unifying themes in viral genome replication or benefit from the advances in other systems. For example, extraordinary biological and experimental paradigms that have been established over the past five years for the DNA replication systems of bacteriophage T4 and T7 will likely be of great value to anyone interested in studying a replisome from any virus. These studies could easily go unnoticed by animal RNA and DNA virologists. It is our hope that this monograph will cross-fertilize and invigorate the field, as well as encourage students into this area of research.