Telomeres, at the end of chromosomes provide genomic stability. During embryonic development, telomerase, a reverse transcriptase elongates the ends of the DNA. In somatic cells, the activity of telomerase decreases after birth leading to shortening of telomere with cell division, which thereby triggers senescence. In embryonic stem cells and germ cells, telomere length is maintained. In adults, the tissue specific stem cells have telomerase activity, but it is not enough to maintain the length of telomere. The stem cells also undergo the process of ageing but it is delayed as compared to the somatic cells. Studies on the genetic disorder, dyskeratosis congenital, caused by mutations in the human telomerase, reiterate the importance of telomere maintenance in human stem cells. This review covers the role of telomere and telomerase in stem cells and their relevance in disease and ageing.