Telomeres are the protective end caps of eukaryotic chromosomes and they determine the proliferative lifespan of somatic cells, as the protectors of cell replication. Telomere length in leucocytes reflects telomere length in other somatic cells. Leucocyte telomere length can be a biomarker of human ageing. The risk of diseases associated with reduced cell proliferation and tissue degeneration, including aging or aging-associated diseases, such as dyskeratosis congenita, cardiovascular diseases, pulmonary fibrosis and aplastic anemia, is correlated with an increase in the shortening of telomeres. On the other hand, the risk of diseases that are associated with increased proliferative growth, including major cancers, is correlated with long telomeres. In most of the cancers, a telomere maintenance mechanism during DNA replication is essential. The reactivation of the functional ribonucleoprotein holoenzyme complex (telomerase) starts the cascade from normal and premalignant somatic cells to advanced malignant cells. Telomerase is overexpressed during the development of cancer and embryonic stem cells, through controlling genome integrity, cancer formation and stemness. Cancer cells have mechanisms to maintain telomeres to avoid initiation of cellular senescence or apoptosis, and halting cell division by critically short telomeres. Modulation of the human telomerase reverse transcriptase is the rate-limiting step for the production of functional telomerase and telomere maintenance. The human telomerase reverse transcriptase promoter promotes its gene expression only in tumor cells, but not in normal cells. Some cancers activate an alternative expansion of telomeres maintenance mechanism via DNA recombination to reduce the shortening of their telomeres. Not only heritability but also oxidative stress, inflammation, environmental factors, and therapeutic interventions have an effect on telomere shortening, explaining the variability in telomere length across individuals. There have been a large number of publications, which correlate human diseases with progressive telomere shortening. Telomere length of an individual at birth is also important to follow up telomere shortening, and it can be used as a biomarker for healthy aging. On the other hand, understanding of cellular stress factors, which affect stem cell behavior, will be useful in regeneration or treatment of cancer and age-associated diseases. In this review, we will understand the connection between stem cell and telomere biology, cancer, and aging-associated diseases. This connection may be useful for discovering novel drug targets and improve outcomes for patients having cancer and aging-associated diseases.