Polo-like Kinase-1 (PLK1) is an important mediator of the G2/M phase of the cell cycle that is down-regulated in a DNA damage dependent manner. In cancer cells PLK1 is overexpressed and allows for continued proliferation of the cell by overriding this checkpoint. Here I show that PLK1 is down-regulated in a p53-dependent manner and that can occur both in response to DNA damage and to a non-genotoxic stimulus of the p53 pathway. My data show that p53 is able to repress PLK1 through a responsive element in the promoter and that p53 is necessary and sufficient to cause PLK1 repression. When examined in the context of a PLK1 promoter/reporter fusion, wild type but not mutated forms of p53 can repress expression. EMSA shows that p53 binds to the p53-responsive element and that mutation of this element reduces p53 binding. Furthermore, PLK1 repression occurs independently of p21-mediated arrest at G1/S, a stage of the cell cycle where PLK1 levels are physiologically low. PLK1 repression mediated by p21 through the CDE/CHR element in the promoter does not appear to cause significant repression of PLK1 but may play a minor role. Down-regulation of PLK1 is relieved by the HDAC inhibitor TSA and supports the transcriptional repression mechanism described in this thesis. Silencing of PLK1 expression by siRNA interferes with cell cycle progression consistent with a role in the p53-mediated checkpoint. This thesis provides two distinct and perhaps overlapping mechanisms by which p53 may repress PLK1: 1)through competitive displacement of an unidentified transcription factor that is essential for normal PLK1 expression and 2) through HDAC recruitment leading to local repression-associated changes in the chromatin structure. These data establish PLK1 as a transcriptional target of p53 that is required for efficient G2/M arrest.