Critical telomere shortening induces senescence in many normal human cell types grown in culture. Recentdata have revealed that dysfunctional telomeres can resemble certain forms of DNA damage, and point to a role for DNA damage signaling in the establishment and maintenance of telomere-initiated senescence. Here, we review these new observations and highlight potential avenues of future research. We consider the identities of the key DNA damage response factors involved in senescence and discuss a model for the molecular events occurring in pre-senescent cells that ultimately lead to a permanent cell cycle arrest phenotype.