p53 is a key transcriptional mediator that controls the expression of hundreds of target genes necessary to maintain cellular homeostasis and genome integrity. An important cellular function that is dependent on p53 transcriptional activity is apoptosis or programmed cell death. Indeed, inhibition of p53 transcriptional activity is often observed in cancers as a result of mutations within its DNA-binding domain. In this chapter, we describe the use of chromatin immunoprecipitation and real-time quantitative polymerase chain reaction to detect p53 transcriptional activity in cancer cells and tumor tissues. This technique enables the determination of the ability of p53 to bind to the promoter region of apoptotic genes and to evaluate the transcription-dependent activity of p53-induced apoptosis.