The tumor suppressor p53 plays a central role in the protection against DNA damage and other forms of physiological stress primarily by inducing cell cycle arrest or apoptosis. Mutation of p53, which is the most frequent genetic alteration detected in human cancers, inactivates these growth regulatory functions and causes a loss of tumor suppressor activity. In some cases, mutation also confers tumor-promoting functions, such as the transcriptional activation of genes involved in cell proliferation, cell survival and angiogenesis. Consequently, cells expressing some forms of mutant p53 show enhanced tumorigenic potential with increased resistance to chemotherapy and radiation. Our current understanding of these activities is summarized in this review. By dissecting out mechanistic differences between wild-type and mutant p53 activities, it may be possible to develop therapeutics that restore tumor suppressor function to mutant p53 or that selectively inactivate mutant p53 tumor-promoting functions.