The studies in this text provide information on the mechanisms by which viruses have evolved together with their natural hosts by acquiring RNA and DNA molecules from the infected cells into their genomes. Part A is devoted to studies on virus genes that were acquired to evade the vertebrate host immune system. Part B deals with the acquisition of cellular and foreign virus genes by Herpes and Irido viruses. The studies presented in Part C describe the poxvirus genes that are homologues of cellular genes. Together, these studies provide an insight into the evolutionary processes that viruses have developed to control the metabolic machinery of the infected tissue cells, and to prevent the defense machinery of the infected host, for example, the immune system, from recognizing the infecting virus. Such mechanisms may explain the pathogenicity and reflect the virulence of viruses.