This chapter explains about Atomic structure (Nucleus, Atomic number, mass number, isomer, isotone, isobar and isotope), Discovery of X-rays (properties, production, Modern X-ray tube, X-ray images), Radioactivity (laws of radioactivity, alpha decay, beta decay, gamma rays, Half life, production and uses of radioisotopes), Radiation protection (Radiation quantities and units, Biological effects, Effective dose limits, Dosimeter, Survey meter and Personnel monitoring systems).