Inactivation of TP53 pathways are the most common defects observed in human cancer. Although missense mutations remain the most frequent genetic event, it is now evident that dysfunction of several members of this network such as MDM2, MDM4 (mdmX), or miR-125b can substitute forTP53mutations. This special issue onTP53brings theTP53gene into the post-genomic era. Several fundamental features of wild type and mutant proteins and their modifications are reviewed, as well as animal models and clinical aspects such as recommendations for patient care. The complex structure of this gene warrants innovative strategies to infer a more accurate status of human tumors. Recommendations and guidelines for reporting and annotatingTP53variants are also provided, to help researchers generate standardized data that are easy to understand, analyze, and exchange across various cancer variant databases.