This year marks two important anniversaries for radioactivity in the fields of biology and medicine: (1) the golden anniversary of the discovery of radium for use in radiation treatment and (2) the silver anniversary of the use of radioactive atoms for tracing stable atoms—that is, of the tracer atom technic. Fifty years ago radium was discovered by Pierre and Marie Curie in France. This led to the wide and valuable use of radioactivity for radiation treatment of certain diseases, particularly cancer. Twenty-five years ago Hevesy in Denmark first used radioactivity to trace the course and behavior of a stable element in a biologic system. This was the inception of the technic of tracing the atoms of stable elements by means of their radioactive counterparts—the radioactive isotopes of that element. The first tracer experiments were done with the naturally occurring radioactive isotopes and were limited to the tracing of the heavy