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 compositions of proteins coded by viral genomes, and searches for regulatory mechanisms in viral nucleic acid replication, as well as identities of motifs in proteins of viruses from all families, will provide additional information on the subject. In future issues that will be devoted to this subject, the origin and evolution of RNA and DNA viruses will be further investigated.