The p53 gene encodes for a protein, p53, which plays a critical role in controlling the cell cycle, in DNA repair and in programmed cell death (apoptosis). p53 is one of the most frequently mutated genes in human neoplasms and a variety of techniques have been developed to detect these mutations. These range from advanced molecular-genetic analyses to immunohistochemical staining for the p53 protein. This review will summarize our current understanding of the function of p53 as well as current methods to detect dysfunctional p53 and the clinical value of such analyses.