The vast majority of p53-regulated genes are induced in response to various stress signals and are responsible for maintaining genetic stability, DNA repair, regulation of crucial cell-cycle check points, and finally induction of apoptosis. The activity of p53 is tightly controlled by two major negative regulators including murine double minute 2 (MDM2; EC 6.3.2.19) and 4 (MDM4 or MDMX) proteins. Human MDM2 and MDMX are structurally related and contain three well-conserved domains: an N-terminal domain (responsible for p53 binding), a zinc-finger domain (function largely unknown) and a C-terminal RING domain (responsible for formation of homo- and heterodimers). Additionally, the RING domain of MDM2 confers E3 ubiquitin ligase activity. Concentration/activity of p53 is kept at low level in unstressed cells. This is accomplished by three parallel mechanisms mediated by MDM2 and/or MDMX. First, MDM2 and MDMX bind the N-terminal transactivation domain (TAD) of p53, preventing thereby its interaction with the transcription machinery and resulting in the inhibition of p53-responsive gene expression. Second, MDM2/X proteins export p53 outside the nucleus into the cytoplasm where it can no longer activate transcription. Finally, MDM2 marks p53 for proteasomal degradation. Many tumors overproduce MDM2 to impair p53 function. Therefore, restoration of p53 activity by inhibiting the p53–MDM2 binding represents an attractive novel approach to cancer therapy.