Telomeres, located at the ends of eukaryotic chromosomes, are synthesized by the enzyme telomerase and are responsible for maintaining chromosome length. The absence of telomerase in most somatic cells has been associated with telomere shortening and aging of these cells. In contrast, high levels of telomerase activity are observed in over 90% of human cancer cells. The absence of telomerase in normal and aging cells is considered a natural defense against development of cancer. However, we do not know what triggers the reappearance of telomerase in cancer cells. Telomerase activity is directly correlated with the expression of its active catalytic component, the human telomerase reverse transcriptase (hTERT), which is believed to be controlled primarily at the level of transcription. Elucidation of the control of telomerase in aging and in cancer as an age-related disease has considerable potential in leading to novel approaches in anti-aging medicine.