On December 2, 1942, man achieved here the first self-sustaining chain reaction and thereby initiated the controlled release of nuclear energy. Thus reads a plaque on the campus of the University of Chicago, where the scientist, Enrico Fermi, supervised the wartime project which led to the development of the atom bomb. reality of controlled nuclear reactions ushered in a new era of society, The Atomic Age, or more correctly, The Nuclear Age. various phenomena associated with radiation, radioactivity, and nuclear energy invQlve changes in the physical constitution and/or energy state of the atomic nuclei. past and present advances in and uses of nuclear energy and radiation are based on the laboratory findings of the past 65 years. Great numbers of men and women from many nations have probed the atom to search out its secrets. Their individual contributions have made possible the present advanced state of nuclear science and technology. biological effect of high energy radiation was not clearly recognized during the early years of radiation studies which began in November, 1895 with Wilhelm Konrad von Roentgen's discovery of X-rays. Reports as early as 1901 note that Henry Becquerel, the discoverer of radioactivity, and Pierre Curie, who was a co-discoverer of polonium and radium, had suffered skin bums from high energy radiations. During and since the first decade of this century, biological damage from X-rays has been further confirmed and studied. Ever-increasing recognition of such effects led to increasingly strict precautions against radiation accidents and injury. During the past twenty years the safety record of nuclear science and industry has on the whole been admirable. Today, study and understanding of biology can be aided by radiation and radioactivity. Radioactive atoms as tracers in biological systems provide information heretofore unavailable. Radiobiological studies benefit medicine, industry, and agriculture. Another important point is that the sciences find unity in radiobiology because the principles of chemistry, physics, and mathematics are as important as those of biology. text and experimental materials of this issue have been prepared as a unit of study for biology students. It is hoped that by intensively investigating the various aspects of radiation and radioactive substances, as well as by learning radiobiological techniques, the student will come to a better understanding of the nature of life processes. Certainly before the student can handle radioactive materials he must be well acquainted with the physical nature of these peculiar atoms. What is a radioactive atom? It is an atom with an arrangement of nuclear components such that it will undergo sudden and spontaneous nuclear change. Associated with this nuclear transformation will be the release of energy and the emission of one or more types of radiation. Radioactivity was discovered among the very heavy atoms. We now know of natural atoms with atomic numbers greater than 80 which are unstable. element thallium, atomic number 81, has several unstable forms. A well-known example of one of these heavy unstable atoms is uranium. atomic number of uranium is 92 and uranium atoms have mass numbers of 234, 235, or 238. number of protons and neutrons in the