The word II radiation II has become a term to fear in our modern world. But what does thisword really mean? Radiation is simply energy transmitted through a distance. There are twotypes of radiation -ionizing and nonionizing radiation. Ionizing radiation (such as x rays andgamma r:ays) causes ionization as it travels through any medium. On the other hand,nonionizing radiation (such as infra-red rays, light, microwaves) does not have sufficientenergy to cause ionization.Ionization is the process whereby an orbital electron acquires sufficient energy to free itselffrom the atom. The remaining positively charged atom is called a positive ion. The removedelectron, either alone or when attached to an electrically neutral atom, is called anegative ion.There are two types of ionizing radiation -electromagnetic and particulate.X and gamma rays are forms of ionizing electromagnetic radiation. X rays are produced invacuum tubes when high velocity electrons strike a target. When these electrons arestopped or slowed down by interaction with atoms of materials in their path, x rays areproduced. On the other hand, gamma rays are emitted when nuclei of radioactive atomsdisintegrate or decay. Another type of ionizing radiation is particulate radiation. Any subatomic particle in motion iscapable of producing ionization. And so we can have protons, neutrons, electrons andeven nuclei of atoms as forms of particulate ionizing radiation as long as they possesssufficient energy to ionize matter. Two forms of particulate radiation have special namesalphaand beta radiation. Alpha rays are nuclei of helium atoms. Beta rays are positivelyand negatively charged electrons. Both alpha and beta rays are emitted by radioactivesubstances.In applications of nuclear energy. ionizing radiation is used. Hence, in this paper, the termradiation will be used to mean ionizing radiation. Agnette P. Peralta, M.Sc.; Bureau of Health Devices and Technology (former name: Radiation Health Service) Department of Health, Manila, Philippines