The study of radiation dosage measurement began twenty-five years ago with the main problem of establishing a system of x-ray dosage. It has been extended in recent years to many other types of radiation. It is important for us today to appreciate the fact that originally the establishment of a unit and method of x-ray dosage was thought to be, or rather hoped to be, a relatively simple and straightforward problem. In the early 20's, x-rays were gaining recognition as a therapeutic agent. It was realized that as an x-ray beam passed through the body it was partially absorbed and it was therefore thought desirable to measure a property of this radiation which was related to its absorption in tissue. It was also recognized that the process of ionization of gases formed a convenient method for measurement. Consequently, the ionization measured in a gas whose effective atomic properties were closely similar to those of tissue was expected to bear a relationship to the ionization produced in tissue. Air meets ...