One in seven types of human cancer is associated with an oncogenic virus infection. Most human tumors have high telomerase activity but very short telomeres, yet the maintenance of these short telomeres is critical to avoid telomere end fusion or senescence and to support active proliferation. Oncogenic viruses have evolved a wide repertoire of strategies to stimulate telomerase functions at the transcriptional and post-transcriptional levels. Since telomerase activity is absent in somatic cells, the inhibition of telomerase is an attractive target for cancer therapeutics.