Following the discovery of artificial radioactivity and the rarer stable isotopes of the light elements, the forty‐year‐old field of applied nuclear physics has found a greatly increased domain of usefulness in chemistry, metallurgy, radiology, geology, physiology and medicine. Isotopic tracer atoms, whether stable or radioactive, provide a powerful method for observing the behavior of complicated biological, chemical or physical systems underequilibriumconditions. Atom smashing machines, when slightly modified, are useful in metallurgical and clinical radiology, while other techniques and results from the field of nuclear physics find important applications in geology and astrophysics.