This volume, derived from Encyclopedia of Virology, provides an overview of the development of virology during the last ten years. Entries detail the nature, origin, phylogeny and evolution of viruses. It then moves into a summary of our understanding of the structure and assembly of virus particles and describes how this knowledge was obtained. Genetic material of viruses and the different mechanisms used by viruses to infect and replicate in their host cells are highlighted. The volume is rounded out with an overview of some major groups of viruses with particular attention being given to our current knowledge of their molecular biology.