Electromagnetic radiation is a form of energy that propagates as both electrical and magnetic waves traveling in packets of energy called photons. There is a spectrum of electromagnetic radiation with variable wavelengths and frequency, which in turn imparts different characteristics. Examples of energy within the electromagnetic spectrum include x-rays (most widely clinically used to treat malignant lesions), visible light, infrared light, and radio waves.The energy of electromagnetic radiation is quantified by an electron volt (eV), where 1 eV describes the energy gained by an electron as it is accelerated through a potential difference of 1 volt. Electromagnetic radiation deposits energy in two forms as it passes through biologic material: excitation and ionization. Excitation describes the deposition of enough energy to raise an electron to a higher electron shell without ejection of the electron. However, ionizing electromagnetic radiation has enough energy to eject one or more electrons from the atom. X-rays and gamma rays, whose properties are equivalent, are clinically the most important form of ionizing electromagnetic radiation in the treatment of cancer.