Multiple pathways are involved in maintaining the genetic integrity of a cell after its exposure to ionizing radiation. Although repair mechanisms such as homologous recombination and nonhomologous end-joining are important mammalian responses to double-strand DNA damage, cell cycle regulation is perhaps the most important determinant of ionizing radiation sensitivity. A common cellular response to DNA-damaging agents is the activation of cell cycle checkpoints. The DNA damage induced by ionizing radiation initiates signals that can ultimately activate either temporary checkpoints that permit time for genetic repair or irreversible growth arrest that results in cell death (necrosis or apoptosis). Such checkpoint activation constitutes an integrated response that involves sensor (RAD, BRCA, NBS1), transducer (ATM, CHK), and effector (p53, p21, CDK) genes. One of the key proteins in the checkpoint pathways is the tumor suppressor gene p53, which coordinates DNA repair with cell cycle progression and apoptosis. Specifically, in addition to other mediators of the checkpoint response (CHK kinases, p21), p53 mediates the two major DNA damage-dependent cellular checkpoints, one at the G(1)-S transition and the other at the G(2)-M transition, although the influence on the former process is more direct and significant. The cell cycle phase also determines a cell's relative radiosensitivity, with cells being most radiosensitive in the G(2)-M phase, less sensitive in the G(1) phase, and least sensitive during the latter part of the S phase. This understanding has, therefore, led to the realization that one way in which chemotherapy and fractionated radiotherapy may work better is by partial synchronization of cells in the most radiosensitive phase of the cell cycle. We describe how cell cycle and DNA damage checkpoint control relates to exposure to ionizing radiation.