Telomerase is a ribonucleoprotein enzyme that binds to the telomeres, increases their length, and extends the lifespan of cells. Telomerase is expressed at low or undetectable levels in normal somatic tissue. However, telomerase is expressed at detectable levels in a wide range of cancer cells, including brain, skin, lung and blood. The selective high level of expression of telomerase in cancer cells makes it an attractive target for cancer therapy. Researchers have investigated cancer therapeutics targeting telomerase and its mechanism of action by multiple approaches, including vaccines, small molecule inhibitors, oligonucleotides and G-quadruplex approaches. Several therapeutics targeting telomerase, such as Imetelstat (GRN163L) and Tertomotide (GV1001), are currently in Phase I, II and III clinic trials to treat a wide range of cancers from pediatric solid tumors to non-small cell lung cancer, leukemia, lymphoma and myeloma.