Cells and tissues respond to reactive oxygen intermediates during physiological processes such as inflammation, ischemia, and reperfusion. Ionizing radiation mimics naturally occurring free radicals to produce similar responses. Radiation induces the production of reactive oxygen intermediates and subsequent oxidation of cell membrane, phospholipids, and DNA. This initial event leads to activation of signals (enzymes) within the cells. These activated enzymes produce a cascade of energy transfer within the cell, leading to activation of transcription factors and the transcriptional apparatus. Radiation-inducible genes are then transcribed, leading in part to biological responses to ionizing radiation. Examples of radiation-inducible genes that regulate the biological response include inflammatory mediators and cell cycle regulators. The products of radiation- inducible genes are proteins such as transcription factors, cell adhesion molecules, and cytokines. The importance of identifying the mechanism of radiation-mediated gene expression is that enzyme inhibitors can be used to prevent gene expression. This synopsis of radiation-induced gene expression reviews genes that are induced by x-rays and y-rays and categorizes them according to their related biological responses.