The TERT (telomerase reverse transcriptase) subunit of telomerase is an intensively studied macromolecule due to its key importance in maintaining genome integrity and role in cellular aging and cancer. In an effort to provide an up-to-date overview of the topic, we discuss the structure of TERT genes, their alternative splicing products and their functions. Nucleotide databases contain more than 90 full-length cDNA sequences of telomerase protein subunits. Numerous in silico, in vitro and in vivo experimental techniques have revealed a great deal of structural and functional data describing particular features of the telomerase subunit in various model organisms. We explore whether particular findings are generally applicable to telomerases or species-specific. We also discuss in an evolutionary context the role of identified functional TERT subdomains.