The data reviewed here suggest that telomere dynamics and telomerase expression are fundamentally involved in cellular aging and cancer. Of particular importance is the stabilization of telomeres by activation of telomerase and the association of this process with cell immortality and human malignancies. Thus, we believe that cell immortalization is required for long-term growth of the vast majority of malignant or metastatic tumors and that advances in telomere biology and telomerase inhibition will improve the way cancers are diagnosed and treated. We look forward to the clinical evaluation of these bold predictions.