This volume, the Desk Encyclopedia of General Virology, is one of a series of four volumes that contain many of the entries in the third edition of the Encyclopedia of Virology edited by Brian W J Mahy and Marc H V van Regenmortel, published by Academic Press/Elsevier in 2008. This first volume consists of 78 chapters that give an overview of the development of virology during the last 10 years. Fifteen chapters discuss general topics such as the nature, origin, phylogeny, and evolution of viruses, and describe the history of virology. The current official taxonomy and classification of viruses, based on the eighth report of the International Committee on Taxonomy of Viruses published in 2005, is also included. The second section of 10 chapters summarizes the considerable progress in our understanding of the structure and assembly of virus particles and describes how this knowledge was obtained using techniques such as X-ray crystallography, cryoelectron microscopy, and image reconstruction. The next section is devoted to the genetic material of viruses and discusses topics such as the exchange of DNA and RNA fragments among parental viral genomes through recombination, the occurrence of retrotransposable DNA elements and defective-interfering RNAs and the newly discovered antiviral role of RNA interference triggered by viral double-stranded RNA.