Cellular senescence is a tumor-suppressive process instigated by proliferation in the absence of telomere replication, by cellular stresses such as oncogene activation, or by activation of the retinoblastoma tumor suppressor protein, pRb. This process is characterized by an irreversible cell cycle exit, a unique morphology, and expression of senescence-associated-beta-galactosidase (SA-beta-gal). Despite the potential biological importance of cellular senescence, little is known of the mechanisms leading to the senescent phenotype. We have recently discovered that expression of active pRb induces expression and altered localization of the ERM family member ezrin, an actin-binding protein involved in membrane-cytoskeletal signaling. pRb expression results in the stimulation of cdk5-mediated phosphorylation of ezrin with subsequent membrane association and induction of cell shape changes, linking pRb activity to cytoskeletal regulation in senescent cells. Cdk5 activity increases in senescing cells and is required for expression of SA-beta-gal and for actin polymerization accompanying acquisition of the senescent morphology. These results begin to illuminate the mechanisms underlying induction of senescence and the senescent shape change and describe new pathways that may contribute to the ability of senescent cells to influence tumor growth.