The studies presented in this special issue of VIRUS GENES provide information on the two aspects of virus evolution: the ancient evolution of viruses from the time prokaryotic and eukaryotic cells evolved, and the ongoing process of the current molecular evolution of viruses. The studies of many scientists collected in this issue and many more that were published in other scientific journals provide insight into the molecular evolution of viruses as one of nature's mysteries. The use of computer porograms to study the nucleotide sequences of viral genomes, the amino acid...