Since the discovery of the enzyme telomerase in humans, it has become apparent that it is the most general of tumour markers known, and enormously significant in its potential for diagnostic, prognostic and therapeutic applications. Extensive work has identified three core components of the enzyme, of which the catalytic subunit hTERT (human telomerase reverse transcriptase) appears to be the most important. The aim of this article is to review the current evidence for the function and activity of hTERT in malignant conditions, and to discuss the future possibilities in terms of cancer diagnosis and treatment. Copyright Harcourt Publishers Limited.