Telomerase is active in early embryonic and fetal development but is down-regulated in all human somatic tissues before birth. Since telomerase is virtually absent or only transiently active in normal somatic cells throughout postnatal life, telomere length gradually decreases as a function of age in most human tissues. Although telomerase repression likely evolved as a tumor suppressor mechanism, a growing body of evidence from epidemiology and genetic studies point to a role of telomerase repression and short telomeres in a broad spectrum of diseases: (a) Humans with shorter than average telomere length are at increased risk of dying from heart disease, stroke, or infection; (b) Patients with Dyskeratosis congenita are born with shortened telomeres due to mutations in telomerase components, suffer from a variety of proliferative tissue disorders, and typically die early of bone marrow failure; and (c) Individuals with long-term chronic stress or infections have accelerated telomere shortening compared to age-matched counterparts. Telomerase activation may prove useful in the treatment of diseases associated with telomere loss. While human cells dividing in culture lose telomeric DNA and undergo changes that mirror certain age- or disease-associated changes in vivo, telomerase transduced cells have extended replicative capacities, increased resistance to stress, improved functional activities in vitro and in vivo, and no loss of differentiation capacity or growth control. In addition, telomerase transduction in vivo can prevent telomere dysfunction and cirrhotic changes in liver of telomerase knockout mice. Thus, pharmacological activation of telomerase has significant potential for the treatment of a broad spectrum of chronic or degenerative diseases.