p53 is a widely conserved tumor suppressor protein that is frequently inactivated in human cancer. p53 functions primarily as a transcription factor regulating the expression of a growing repertoire of target genes. p53 integrates signals from many stress-activated pathways and is subject to multiple posttranslational modifications. Phosphorylation and acetylation have been implicated in the regulation of p53 stability and activity. In response to DNA damage, hypoxia, oncogene activation and other types of stress, activated p53 triggers a variety of cellular programs, often in a stimuli- and cell type-specific manner. In particular, the role of p53 in cell growth arrest and apoptosis is criticial for its tumor suppressor activity.