During the passage of radiation through matter comes to their interactions, which resulted in changing the properties of matter and radiation. Ionization of atoms is the most important effect which is caused by the radiation, so it is called ionizing radiation. Direct ionization occurs when electrically charged particles or photons from neutral atoms remove an electron. The result of this expulsion are free negative electrons and positively charged atoms. Indirect ionization occurs when neutral particles or photons from atoms remove an electron and the electron has enough energy to further ionize atoms. Indicator of the degree of ionization is the number of ionized pairs created by the particles or photons passing through a particular matter. During passage of electromagnetic radiation through the matter usually couses photoelectric effect, Compton effect and the formation of electron-positron pairs. Photoelectric effect is phenomenon which can appear when energy of radiaton photon is greater than the binding energy of electrons in atom. Photon emits all the energy to the electron, thus the electron leaves the atom and the photon stop to exist. Compton effect is a phenomenon in which a photon transmit only part of its energy to an electron, and continues to go further with another direction and with less energy values. The formation of an electron-positron pair is a phenomenon that occurs only if the photon energy is greater than 1.2 MeV. In case where photon penetrat to the nucleus, in its vicinity turns into two particles - electrons and positrons. After a very short time, the positron combines with some of the electrons from the environment and thereby both particles disappear. As a result of their disappearance two photons with opposite directions of motion are formed. The probability of occurrence of one of these phenomena depend on the energy of the incident photons and particles. At low energies occurs mainly photoelectric effect, Compton effect dominated at higher photon energies, and at photon energies greater than about 5 MeV predominant is formation of pairs. Effects of ionizing radiation on the human body can occur by direct action on the macromolecules of the body or by indirectly acting on the molecules of water (radiolysis) whose free harmful radicals damage other molecules of the body. Irradiation damage and symptoms can manifest soon after radiation exposure or after a longer period. Radiation can cause carcinogenesis, leukemias damage, teratogenic and genetic damage. Some diseases can be caused by the accumulating of radiation through numerous exposures, in which irradiated cells failed to recover. Also, damage may appear as a result of minor doses, but those doses are large enough to affect the normal growth of cells and their multiplications. Most sensitive cells are not well differentiated and those which reproduce rapidly. Ionizing radiation should be used carefully, professionally and conscientiously, bearing in mind protection for all who are exposed. There are many organizations, laws and principles that indicate the importance of careful handling with radiological devices. Professional staff should educate patients about the potential risks caused by ionizing radiation and radioactive sources. Special categories of protection are pregnant women and young children for whome the smallest doses of radiation can be very dangerous. The population is constantly exposed to radiation because it is present in many natural resources, but we can reduce the overall recived doses by reducing unjustified exposure to radiological diagnostic and therapeutic procedures, taking into account the objective justification and need for examination in relation to the harm and danger of radiation.