Radiation biology refers to all biologic responses induced in cells and tissues by ionizing radiation (IR), a term that encompasses the study of all action of ionizing radiation on living things. IR causes the ejection of an orbital electron when the radiation interacts with tissue. IR interacts with all parts of the cell, although the creation of unrepairable DNA double‐strand breaks is believed to be the lethal event. Modern radiobiology has come to recognize that cells not directly hit by ionizing events still show effects via the bystander mechanism. The exact mechanism of that remains unclear, but it can be mediated through gap junctional communication or through the transmission of soluble factors released into the extracellular space. At the tissue level, the ultimate response to IR events depends on the “4 R’s of radiobiology”: repair, repopulation, redistribution, and reoxygenation. Radiation dose response is believed to follow a linear no‐threshold model. Although initial damage seems to follow this pattern, a gap in the scientific literature remains on how repair and downstream effects are treated in the cell or tissue at the lowest exposure levels.