Telomerase is a specialized reverse transcriptase that is responsible for extending and preserving the end of the chromosomes (telomeres). Telomerase plays a key role in regulating the lifespan of mammalian cells and is involved in critical aspects of cellular ageing processes. In this review, we will briefly summarize our current understanding of the functions of telomeres, telomerase and their regulation. Considering that compensatory islet hyperplasia and beta-cell regeneration play important roles in the prevention and/or delay of the onset of overt diabetes, we will also examine current literature regarding the effects of diabetes on telomere shortening and provide insights from our own studies on the role of telomerase in beta-cell regeneration.