In the United States there is a slow migration toward the use of the Systeme Internationale (SI) nearly universally employed elsewhere. This requires the practitioner to understand the interconnections of the units, both those “customary” in the United States and SI, due to the diversity found in both the scientific literature and government regulations. Standards or limits on occupational and environmental exposure to ionizing radiation are instituted worldwideTo develop an understanding of accelerator radiation physics, it is necessary to introduce the quantities of importance and the units by which they are measured. Over the years various systems of units have been employed. In general, individual nations, or subnational entities, incorporate guidance provided by international or national bodies into their laws and regulations. The main international body that develops radiological standards designed to promote occupational and environmental radiological health is the International Commission on Radiological Protection.