Approximately, 1.4 million virus-induced cancers occur annually, representing roughly 10% of the worldwide cancer burden, with the majority (> 85%) occurring in the lower- and middle-income countries. The viruses associated with the greatest number of cancer cases are human papillomaviruses (HPVs), which cause cervical cancer and several other epithelial malignancies, and hepatitis viruses HBV and HCV, which are responsible for the majority of hepatocellular cancer. Other oncoviruses include Epstein-Barr virus (EBV), Kaposi's sarcoma herpesvirus (KSHV), human T-cell leukemia virus (HTLV-I), and Merkel cell polyoma virus (MCPyV). These oncoviruses include various classes of DNA and RNA viruses and induce cancer by a variety of mechanisms. However, cancers develop in a minority of infected individuals and almost always after chronic infection of many year's duration. Identification of the oncoviruses has provided critical insights in human carcinogenesis and led to several interventions that may reduce the risk of developing the tumors they induce. These interventions include preventive vaccines against HBV and HPV, screening for persistent HPV and HCV infections, antivirals for the treatment of chronic HBV and HCV infection, and screening the blood supply for the presence of HBV and HCV. Further efforts to identify additional oncogenic viruses in human cancers and new insights into etiology and pathogenesis of virally induced cancers would likely lead to new approaches for prophylactic and therapeutic interventions.