In vivo and in vitro evidence indicate that cells do not divide indefinitely but instead stop growing and undergo a process termed cellular proliferative senescence. Very little is known about how senescence occurs, but there are several indications that the retinoblastoma protein (pRb) is involved, the most striking being that reintroduction of RB into RB^−/−tumor cell lines induces senescence. In investigating the mechanism by which pRb induces senescence, we have found that pRb causes a posttranscriptional accumulation of the cyclin-dependent kinase inhibitor p27^KIP1 that is accompanied by an increase in p27^KIP1 specifically bound to cyclin E and a concomitant decrease in cyclin E-associated kinase activity. In contrast, pRb-related proteins p107 and p130, which also decrease cyclin E-kinase activity, do not cause an accumulation of p27^KIP1 and induce senescence poorly. In addition, the use of pRb proteins mutated in the pocket domain demonstrates that pRb upregulation of p27^KIP1 and senescence induction do not require the interaction of pRb with E2F. Furthermore, ectopic expression of p21^CIP1 or p27^KIP1 induces senescence but not the morphology change associated with pRb-mediated senescence, uncoupling senescence from the morphological transformation. Finally, the ability of pRb to maintain cell cycle arrest and induce senescence is reversibly abrogated by ablation of p27^KIP1 expression. These findings suggest that prolonged cell cycle arrest through the persistent and specific inhibition of cdk2 activity by p27^KIP1 is critical for pRb-induced senescence.