The acknowledgment that viruses are potent biological factors in driving many cancers have seen a dramatic upsurge in recent years in large part to the success of the human papilloma virus vaccine against invasive cervical carcinomas and followed by the awarding of the noble prize in medicine in 2008 to Dr. Harald zurHausen who identified the link between papilloma virus and cervical cancers. Over the last few years there have been some volumes addressing different aspects of viruses and cancers and to some extent focusing on the DNA viruses, more specifically the human DNA viruses. This proposed volume will attempt to review and address the major gaps in current knowledge in DNA viruses as well as RNA viruses bringing a historical perspective of where studies began to a more recent molecular approach and vaccine successes in tumor viruses. We will also cover other known oncogenic viruses associated cancers in other mammals in addition to humans.