Oncogenic viruses are important pathogens in farm and companion animals. These original pathogens are classified in various virus families, such as Retroviridae, Papillomaviridae, and Herpesviridae. Besides a role as pathogens for its original host, animal viruses serve as valuable models for viruses affecting humans, such as hepatitis B virus, and issues of immunity, therapy, but also basic pathophysiological mechanisms, can often only be addressed in those animal systems.