The viruses of Archaea are likely to be useful tools for studying host evolution, host biochemical pathways, and as tools for the biotechnology industry. Many of the viruses isolated from Archaea show distinct morphologies and genes. The euryarchaeal viruses show morphologies similar to the head-and-tail phage isolated from Bacteria; however, sequence analysis of viral genomes from Crenarchaea shows little or no similarity to previously isolated viruses. Because viruses adapt to host organism characteristics, viruses may lead to important discoveries in archaeal biochemistry, genetics, and evolution.