The role of viruses in the etiology of animal cancers is fairly certain. Information derived under both natural and experimental conditions supports the concept that either DNA- or RNA-containing viruses can fulfill this function. The DNA-containing herpesviruses, especially the Epstein-Barr virus, are currently the primary objects of intense investigation concerning their role in human cancer. This article will focus on the properties of counterpart herpesviruses in lower animals as well as the human virus candidates with an assessment of the observations concerning their oncogenic potential.