The paper gives a review on features of telomeres with particular emphasis on a correlation between telomeres and organism sensitivity to ionizing radiations and their potential importance in radiation biology. Recent findings show that radiosensitivity can be impacted by changes in telomere length or telomere-being locations. Shortened and dysfunctional telomeres join to DNA breaks interfering with their correct repair. Chromosome instability resulting from telomere loss plays a significant role in chromosomal rearrangements associated with tumor progression. Telomere function is one of the factors that determine radiosensitivity of cells and whole organism.