Since the discovery of the tumor suppressor p53 and the Ras oncogene, ample data have been accumulated, describing their aberrations in human cancer and their contribution to the multistep process of tumorigenesis. Several studies have also demonstrated that these dysregulated pathways cooperate to promote malignancy. Here we review recent studies on the cooperative molecular mechanisms by which p53 inactivation and oncogenic Ras converge to enhance tumorigenesis.