It would be far better if we could find a way in which the radioactive by-products could be made to serve a useful and safe purpose. Some of the by-products can be used and have been used. These uses are connected with some hazards. Furthermore, only a small fraction of the fission products have found good employment up to the present. But the importance of fission products is growing. We are using them in research. A radioactive isotope imitates the behavior of its non-active brother in all chemical reactions and in all the intricate processes in which matter changes its form inside a living body. Furthermore a radioactive substance can be detected with the greatest ease. It can be found in a concentration which is less than a millionth of a safe radiation dose. What the microscope has been in the exploration of the structure of organisms, the radioactive elements may become in the understanding of the chemical functioning of living matter. With better understanding there comes the possibility of using radioactive by-products for diagnosis. As with the medical use of X-rays the possible small damage due to radiation exposure should be regarded as the price for the help we can get from early and correct recognition of diseases. In the treatment of patients, particularly in the case of persons stricken by cancer, radioactive destruction of the diseased tissue is often preferable to the use of the surgeon’s knife. Such radioactive treatment is new. There is much room for improvement. Appropriate use of radioactive substances for this purpose may become a far more powerful tool and much more widespread than it is at present. But all these applications will use up only a vanishing fraction of the fission products. Moreover, most of the biologically important elements are not produced in the fission of uranium. Many useful activities can be produced by neutron absorption in reactors. But among the fragments of uranium perhaps only radio-iodine has been put so far to direct physiological use. Industry deals with less sensitive objects than living tissue. Therefore greater amounts of radioactive materials can be used here. And indeed radioactivity has done a great variety of jobs. The penetrating power of X-rays has been used to control the thickness of sheets in an easy and automatic manner. Radioactivity has been incorporated into surfaces which are exposed to mechanical wear or corrosion, to check the rate at which the surface is worn away by the appearance of activity in the lubricant or other fluids which have been in contact with the surface. By such methods industry has accumulated savings which are rapidly approaching the billion dollar mark. These savings will increase as people learn how to use the new materials. But in all these cases it is important to make sure that the activity will not hurt anyone while it is used and after it has served its purpose.