This chapter addresses challenges at the ends of linear eukaryotic chromosomes, explaining how these challenges originate in DNA structure, its directionality and replication mechanisms. I introduce the two major challenges posed by linear chromosomes: the end replication problem, which leads to progressive shortening with each cell division, and the end protection problem, where chromosome ends must be shielded from being recognised as DNA damage. I discuss in detail the specific structure of telomeres, their shortening during cell divisions and their maintenance by telomerase. I explore the vital role of telomerase in maintaining telomere length in stem cells, its implications for ageing and its dysregulation in cancer. Finally, I examine hypotheses regarding the evolutionary origin of linear chromosomes and telomeres.