This lecture covers the role of telomeres in cancer biology, focusing on telomerase, telomere replication problems, and the alternative lengthening of telomeres (ALT) pathway. It discusses the historical perspective, telomere function, end replication problems, telomerase reaction cycle, telomeric DNA, and the Nobel Prize-winning discovery of telomeres protection. The lecture also explores the end replication problem solutions, telomere maintenance mechanisms, and the impact of telomeres on cellular senescence and tumorigenesis.