EM radiation is a form of energy propagation where photons with both particle and wavelike properties travel at the speed of light.1 EM waves carry energy and transfer their energy upon interaction with matter. The energy associated with EM radiation is proportional to frequency and inversely proportional to wavelength. Thus, EM waves with shorter wavelengths have more energy. Examples of EM radiation (from lowest to highest energy) include radio waves, microwaves, infrared, visible light, UV, and radiographs (Fig. 3.1). EM radiation can be further divided into ionizing and nonionizing radiation. EM radiation at or below the UV spectrum is nonionizing, whereas radiographs are ionizing. Ionizing EM has enough energy to remove tightly bound electrons from an atom or molecule. The release of bound electrons leads to the generation of ions and free radicals. Within living cells, ions and free radicals interact with cellular machinery and cause DNA damage, which can ultimately lead to cell death. Radiotherapy uses ionizing radiation in order to cause damage to tumor cells. The photons used for therapeutic radiation generally have wavelengths of 10−11 to 10−13 m.