The p53 tumor suppressor protein is extensively post-translationally modified, mostly by phosphorylation. The phosphorylation sites are clustered into two distinct domains within the p53 polypeptide and the protein kinases and phosphatases which modify many of these sites have been identified. In addition, signaling pathways which modulate the phosphorylation state of p53, leading perhaps to changes in its activity, are being actively investigated. Similarly, the transforming proteins of DNA tumor viruses modulate p53 phosphorylation and may therefore be useful tools for probing these regulatory mechanisms. Given the very potent effects of p53 on cell growth and the extent of phosphorylation of this protein, p53 may well be controlled tightly and coordinately by more than one signaling mechanism.