Ionizing radiation is a threat to human cells and ultimately to the entire organism. This threat begins with the induction of DNA damage, which may then be converted into permanent genetic change. Ionizing radiation is an activator of onocogenes and an inactivator of tumor-suppressor genes. Together, these genetic events, when induced in cells that still grow and divide, can lead to unregulated cellular proliferation and cancer. Most single-strand breaks and base alterations can be repaired perfectly. Even double-strand breaks are repaired. Our understanding of radiation’s role in its induction of acute & chronic effects is far from complete. We have only recently begun to appreciate the remarkably active role that the cell plays effectively in preventing the damage by ionizing radiation. Current and future research into the cell cycle, the regulation of cellular proliferation, and the impact of radiation on these processes will undoubtly lead to additional insights. Each of these discoveries will move us closer to understanding the aberrations in cellular metabolism that enable the cellular change.