Shortening of telomeres occurs with cell proliferation and correlates well with ageing in humans. Telomerase is a ribonucleoprotein, and is the body's most widely studied mechanism for extension of telomeres to circumvent cellular ageing. Telomerase levels remain at low or unmeasurable levels in most somatic cell populations with only a few exceptions. However, in transformed cell populations, upregulation of telomerase or some other mechanism for telomere extension is required for immortality. The telomere-telomerase system has been proposed to be an adaptation of organisms with prolonged lifespan to avoid malignant tumors, at the cost of the cellular dysfunctions associated with the aged phenotype. Therapies to modulate telomere length and telomerase levels hold promise for therapy of cancer and ageing as well as for genetic conditions that predispose to an aged phenotype.