Main purpose of this lecture is to present on Units of Radioactivity. Some radio-nuclides however survive to eventually reach the surface of the earth. Among these are H (tritium), Be (beryllium-7) and C (carbon-14) which has the longest half-life (5730 years). The original unit for measuring the amount of radioactivity was the curie (Ci)–first defined to correspond to one gram of radium-226 and more recently defined as: 1curie = 3.7×1010 radioactive decays per second. 1 becquerel = 1 radioactive decay per second = 2.703×10–11 Ci.