Ionizing radiations are capable of damaging biological tissues, with the nature of damage inflicted being related to the type, energy and penetrating power of the radiation involved. There are essentially two kinds of ionizing radiations: electromagnetic radiations such as x-rays and gamma rays, and corpuscular radiations, such as, alpha particles, beta particles (electrons), protons, neutrons, etc. The main natural sources of ionizing radiation in the environment are cosmic rays, and the decay of radioactive elements in rocks, soils, waters and atmosphere. The ionizing radiation to which man is exposed is derived from two sources: External, arising from cosmic rays, and from primordial radionuclides present in the earth’s crust, in the building materials and in the air, and internal, arising from naturally occuring nuclides taken into the human body through food and water. Apart from x-rays, radiations from a number of radio-isotopes are used for diagnostic purposes.