Telomeres, the natural ends of linear eukaryotic chromosomes, are essential for protecting chromosomes from degradation and fusion. The synthesis of telomere DNA repeats in most eukaryotes is performed by a special enzyme, telomerase. Telomerase, a ribonucleoprotein enzyme, is a specialized reverse transcriptase utilizing its RNA moiety as a template for synthesis of telomeric DNA. Enzymatic properties and results of comparative analysis of telomerase RNA and protein structures from different eukaryotic systems are discussed in this review.