In the mid-20th century, Leonard Hayflick discovered that normal human cells have a limited number of divisions, which became a significant contribution to the study of replicative aging. Together with Paul Moorhead, he demonstrated that the replicative limit of cells is a fundamental biological feature. Their research led to the concept of the replicometer and the idea of telomeres as a mechanism that restricts cell divisions. Alexey Olovnikov proposed a hypothesis explaining the shortening of telomeres during each replication cycle. The discoveries of Hayflick, Olovnikov, and Blackburn laid the foundation for research into replicative immortality, which is of critical importance for understanding replicative aging and cancer. Research has identified the existence of two classes of cells—replicatively mortal and replicatively immortal. To this day, the replicometer remains to be discovered.