Viruses that produce tumours are defined as oncogenic viruses. These include Papovaviridae; Papillomavirus (some); Herpesviridae Lymphocryptovirus (Epstein-Barr virus); Hepadnavirus; Hepatitis B virus(HBV); Adenoviridae; Poxviridae; Human T-cell leukaemia virus (HTLV-1; HTLV-2); Sarcoma viruses of cats, chickens, rodents; Mammary tumour virus of mice and Feline leukaemia virus (FeLV). This book presents leading-edge research in this field.