Telomeres, the ends of linear chromosomes, shorten with each round of DNA replication. Loss of telomeric DNA can lead to senescence, a state in which cells no longer divide, and crisis, which triggers cell death. To prevent these phenomena, cancer and stem cells must maintain their telomeres, for example, by expressing telomerase, an enzyme that can extend telomeres. As our knowledge of telomere maintenance increases, opportunities arise for translating telomere biology into clinical medicine. Areas of current investigation include the development of diagnostic and prognostic markers for cancer; the development of chemotherapeutic agents based on telomerase inhibition, an immune response to telomerase, or telomerase-based gene therapy; and engineering rejuvenated tissues by restoring telomerase expression.