p53 is one of the most important and most frequently mutated tumor suppressors in human cancers and as such has been intensively studied for a long time. p53, which also has roles beyond cancer in normal tissues, is a major orchestrator of the cellular response to a broad array of stress types by regulating apoptosis, cell cycle arrest, senescence, DNA repair and genetic stability. These diverse actions of p53 rely on its function as a transcription factor, as well as on its transcription independent cytosolic functions. An example of transcription independent p53 function is the direct mitochondrial p53 pathway where p53 regulates the intrinsic apoptotic machinery by several direct interactions with members of the Bcl-2 family. Today, an exciting venue in p53 research is the development of p53-reactivating compounds such as Nutlin for the treatment of human cancers that retained wtp53. Although Nutlin-type compounds hold great promise, improvements are still necessary before they can be established in the clinic. During my dissertation research I conducted three studies related to p53. The first shows a critical role of the direct mitochondrial p53 pathway during Nutlin-induced apoptosis in wild-type p53 retaining cancer cells, therefore establishing this pathway as a therapeutically relevant mechanism in p53-based therapies. My second project identifies the Hsp90 inhibitor 17AAG as a potent synthetic lethal synergistic partner of Nutlin for treatment of difficult-to kill solid human tumors. My third project describes a completely novel and exiting function of mitochondrial p53 in inducing tissue necrosis during oxidative stress. Upon such stress, p53 translocates to the mitochondrial matrix and regulates (induces opening of) the mitochondrial Permeability Transition Pore by direct interaction with the pore's essential regulatory protein Cyclophilin D. This unsuspected new function of p53 impacts brain ischemia-reperfusion injury during stroke in vivo.