Components of telomere maintenance implicated in human disease. Telomeres are composed of thousands of TTAGGG repeats localized at the ends of linear chromosomes. Telomeres are coated by shelterin, a 6-protein complex (TRF1, TRF2, TIN2, POT1, TPP1, and RAP1) with multiple roles in maintaining telomere length homeostasis by forming the T loops, preventing DNA damage response activation, and recruiting the telomerase complex and modulating its activity. The 3′ end of the telomeric leading strand terminates as a single-stranded overhang, which folds back and invades the double-stranded telomeric helix, forming the T loop. Unwinding of the T loop, needed for telomere elongation, is done by a DNA helicase, RTEL1. Telomerase, the enzyme responsible for telomere elongation, has a 4-protein scaffold (dyskerin, NOP10, NHP2, and GAR) and an RNA template (TERC) and reverse transcriptase TERT. TCAB1 ensures trafficking of the telomerase complex to the telomeric ends. The CST complex has multiple proposed roles through its 3 members (CTC1, STN1, and TEN1); CTC1 inhibits telomerase activity and also promotes lagging strand synthesis by binding single stranded DNA and interacting with α polymerase primase. Components in gray have not been implicated in human disease.