The compounds or substance which emit radiation and which are used in medicine area called radio pharmaceuticals. Radio opaque substances are those compounds (both inorganic and organic) that have the property of casting a shadow on X-ray films. Radio pharmaceuticals preparations are the preparations containing one or more radio nuclides. Radio nuclides are radioactive and transform spontaneously into other nuclides. The unstable nuclei undergo spontaneous nuclear changes in order to achieve stability by emitting radiation are called radioisotopes or radio nuclides. The radioactive substances disintegrates or decays with the emission of certain particles or certain quantities of energy which is always characteristics of the isotope. This book gives various ideas about radiations, radioactivity, radioisotopes, radio opaque and contrast media,radio pharmacology, radioactive pollution as well as quality control and also pharmaceutical applications of radioisotopes.