Telomerase is an important enzyme required for maintenance of chromosome ends (telomeres) during cell division. Since elevated activity of telomerase has been selectively demonstrated in a large number of human cancers, it has generated considerable excitement as a potential cancer marker, a prognostic factor, and a target of possible anticancer strategy. This article details diagnostic, prognostic, and therapeutic prospects linked to the measurement of telomerase activity and the growing complexity of its regulation and function.