Exposure of a wide variety of cells to ionizing (X- or gamma-) irradiation results in a division delay which may have several components including a G1 block, a G2 arrest or an S phase delay. The G1 arrest is absent in many cell lines, and the S phase delay is typically seen following relatively high doses (> 5 Gy). In contrast, the G2 arrest is seen in virtually all eukaryotic cells and occurs following high and low doses, even under 1 Gy. The mechanism underlying the G2 arrest may involve suppression of cyclin B1 mRNA and/or protein in some cell lines and tyrosine phosphorylation of p34cdc2 in others. Similar mechanisms are likely to be operative in the G2 arrest induced by various chemotherapeutic agents including nitrogen mustard and etoposide. The upstream signal transduction pathways involved in the G2 arrest following ionizing radiation remain obscure in mammalian cells; however, in the budding yeast the rad9 gene and in the fission yeast the chk1/rad27 gene are involved. There is evidence indicating that shortening of the G2 arrest results in decreased survival which has led to the hypothesis that during this block, cells repair damaged DNA following exposure to genotoxic agents. In cell lines examined to date, wildtype p53 is required for the G1 arrest following ionizing radiation. The gadd45 gene may also have a role in this arrest. Elimination of the G1 arrest leads to no change in survival following radiation in some cell lines and increased radioresistance in others. It has been suggested that this induction of radioresistance in certain cell lines is due to loss of the ability to undergo apoptosis. Relatively little is known about the mechanism underlying the S phase delay. This delay is due to a depression in the rate of DNA synthesis and has both a slow and a fast component. In some cells the S phase delay can be abolished by staurosporine, suggesting involvement of a protein kinase. Understanding the molecular mechanisms behind these delays may lead to improvement in the efficacy of radiotherapy and/or chemotherapy if they can be exploited to decrease repair or increase apoptosis following exposure to those agents.