Radiation dosimetry has made notable progress since the early days of the use of x-rays. The adoption in 1928 of the roentgen as a unit of dose based upon the ionization produced by x-rays in air, followed by many classical dose distribution studies, changed the application of ionizing radiation to biological material from a qualitative to a quantitative science. In more recent years, high-energy photon beams, particle beams, and radioactive isotopes have been added to the armamentarium of the radiation worker. These new sources of radiation emphasized the inherent limitations of the roentgen as a unit of dose and, consequently, in 1953 the International Commission of Radiological Units adopted the more general concept of absorbed dose, the unit for which is the rad. The definition of the roentgen was unchanged, but its use was restricted to x-rays and gamma-rays below 3 MEV in energy and the new term “exposure dose” was coined to identify the concept expressed by the roentgen. The roengten and the rad ar...