Telomere damage resulting from telomere shortening can potentially suppress tumorigenesis by permanently arresting or eliminating incipient cancer cells. Dysfunctional telomeres activate the canonical DNA damage response pathway, resulting in a p53-mediated G(1)/S arrest and senescence or apoptosis. Experimental induction of telomere damage through inhibition of the telomeric protein TRF2 recapitulates aspects of telomere attrition, including a p53-mediated cell cycle arrest. Using this system, we have shown that telomere damage can also elicit a G(1)/S arrest through the RB-regulator p16INK4a, especially in cells lacking p53 function. Here we discuss the significance of p16INK4a as a second effector of the telomere damage response.