Cellular senescence is a state of irreversible cell cycle arrest, accompanied by and in most cases driven by a persistent DNA damage response (DDR), which may be activated by uncapped telomeres or other forms of DNA damage. DNA damage foci, therefore, are an important part of the signaling pathway that induces cell senescence. however, similar foci can also be observed in proliferating cells, for instance as a result of replicative stress. Identifying the phenotypic differences between the DDR of young, proliferation-competent cells and senescent cells is therefore important for establishing the cellular DDR as a marker of senescence. Here, we describe various methods by which the DDR can be used as a robust marker of cellular senescence, and how to utilize a DDR marker to investigate the induction and stabilization of the senescent phenotype.