Telomerase, a ribonucleoprotein enzyme which can compensate for the erosion of telomeric repeats each cell division, is considered to be associated with cellular immortality. The genes encoding the major components of human telomerase, TERC for telomerase RNA component, TEP1 for a human homolog of p80, and TERT for catalytic subunit, have been cloned. In addition, TERF1 and TERF2, encoding telomeric repeat binding proteins, have been cloned and shown to be involved in telomere length regulation and the protection of chromosome end fusions, respectively. Although the activity level of telomerase is similar at each stage of the cell cycle in telomerase-positive cells, it is generally upregulated with cellular proliferation and repressed with withdrawal from cell cycle. Although biological mechanisms regulating telomerase activity and telomere length remain elucidated, telomerase activity can be detected in approximately 85% of human malignant neoplasms, and the telomerase is a promising and novel tumor marker for clinical diagnosis of cancer, as well as a novel target of anticancer therapy.