Ribonucleoprotein telomerase is an enzyme that elongates telomeric DNA. In cells without detectable telomerase activity telomeres shorten with every cell generation and reaching critical length is a signal for cell death. Normal human somatic cells express undetectable, or low (bone marrow and peripheral leukocytes), telomerase activity. Reactivation of telomerase (immortalization) is probably necessary during development of a fully malignant cancer. Consequently, telomerase was proposed to be a therapeutic target for the cancer therapy. Potential results (including side-effects) of telomerase inhibition are being considered. After all, telomerase inhibition can be useful not only in the therapy, but also in cancer biology research, elucidating ageing and immortalization phenomena.