Telomeres are protective structures at the ends of linear chromosomes. They are critical at the cellular level for maintenance of the genome stability. At the organismal level they may contribute to ageing, which is considered to be tumor supressing mechanism. They can regulate the lifespan of cells and protect natural chromosome ends from unwanted DNA repair activities, such as non-homologous end joining and homologous recombination. The chromosomes end in telomeric DNA containing tandem hexanucleotid repeats in specific loop structure. The complex of specific proteins, named Shelterin, takes part in the maintenance of the T loop and telomere function. Shelterin consists of six proteins: TRF1, TRF2, TIN2, Rap1, TPP1 and POT1. The complex has a high affinity toward telomeric sequence. TRF2 inhibition compromises the structure of the telomeres and can cause p53 activation, apoptosis or cell senescence as well as chromosome end fusion, depending on the cell type. The recent investigations had identified telomeric repeat containing RNA, TERRA, a large noncoding RNA forming an integral component of telomeric heterochromatin. TERRA functions suggest important roles in the regulation of telomerase and in orchestrating chromatin remodelling throughout development and cellular differentiation.